Feature/metk 176 port pymetkit to pybind11

[tbkr]

Description

Porting the existing pymetkit layer to pybind11

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

📋 Metkit Documentation 📋
https://sites.ecmwf.int/docs/metkit/pull-requests/PR-239

[Copilot]

Pull request overview

This PR ports the pymetkit Python layer from the legacy CFFI approach to a new pybind11-based extension module, adds a small Pythonic wrapper API, and wires packaging/tests/docs into the CMake and CI workflows.

Changes:

• Add pymetkit_bindings (pybind11) + a Pythonic pymetkit layer (MarsRequest, MarsSelection, parse_mars_request) built and packaged from the CMake build.
• Add a new tests/pymetkit pytest suite (integration tests for request construction/access/expand/merge/parse + CLI tests) and hook it into ctest.
• Add/refresh docs/pymetkit for installation, API, bindings, examples, and legacy migration notes; mark the old CFFI code as deprecated.

mars2grib documentation sync (CodingGuidelineID: 1000000):

• Impacted files under src/metkit/mars2grib: none
• Impacted files under tests/mars2grib: none
• Documentation status for these paths: ✅ in sync (no affected files)

Reviewed changes

Copilot reviewed 40 out of 40 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/pymetkit/integration/mars_request/test_selection.py	Adds integration tests for MarsSelection normalization and validation.
tests/pymetkit/integration/mars_request/test_parse.py	Adds integration tests for parsing requests from strings/files.
tests/pymetkit/integration/mars_request/test_merge.py	Adds integration tests for request merging semantics and error cases.
tests/pymetkit/integration/mars_request/test_expand.py	Adds integration tests for expansion/default inheritance and validation.
tests/pymetkit/integration/mars_request/test_equality.py	Adds integration tests for equality and repr behavior.
tests/pymetkit/integration/mars_request/test_construction.py	Adds integration tests for construction sources and value normalization.
tests/pymetkit/integration/mars_request/test_accessors.py	Adds integration tests for mapping-like access/mutation APIs.
tests/pymetkit/CMakeLists.txt	Registers pytest tests with CTest and configures test environment.
tests/pymetkit/cli/test_cli.py	Adds tests for the python -m pymetkit diagnostic CLI behavior/logging.
tests/CMakeLists.txt	Conditionally includes tests/pymetkit when the feature is enabled.
src/pymetkit/README.md	Documents architecture/usage for the new pybind11-based package.
src/pymetkit/pymetkit.py	Implements Pythonic MarsRequest wrapper and parse_mars_request.
src/pymetkit/pymetkit_type.py	Implements MarsSelection with normalization helpers.
src/pymetkit/_internal/pymetkit_internal.py	Initializes bindings and defines MetKitException.
src/pymetkit/_internal/init.py	Loads libmetkit via findlibs and re-exports binding symbols.
src/pymetkit/main.py	Adds CLI for dependency/home/version diagnostics.
src/pymetkit/init.py	Defines the public import surface (MarsRequest, etc.).
src/pymetkit_bindings/CMakeLists.txt	Builds the pybind11 extension and stages the output.
src/pymetkit_bindings/bindings.cc	Implements pybind11 bindings for metkit::mars::MarsRequest + parsing/version info.
src/CMakeLists.txt	Adds staging layout + wheel build target for pymetkit.
python/pymetkit/tests/test_marsrequest.py	Minor formatting tweaks in legacy tests file.
python/pymetkit/src/pymetkit/pymetkit.py	Marks legacy CFFI module as deprecated (docstring).
python/pymetkit/README.md	Marks legacy CFFI interface as deprecated and points to new package/docs.
pyproject.toml	Removes the legacy pyproject-based packaging configuration.
docs/pymetkit/requirements.txt	Adds Sphinx/doc build requirements for PyMetKit docs.
docs/pymetkit/legacy.rst	Documents the legacy CFFI interface and migration notes.
docs/pymetkit/installation.rst	Documents build/install flows and findlibs diagnostics.
docs/pymetkit/index.rst	Updates landing page and adds quick start/toctree structure.
docs/pymetkit/examples.rst	Adds doctestable usage examples for the new API.
docs/pymetkit/development.rst	Adds development workflow notes (wheel building, env vars, docs).
docs/pymetkit/conftest.py	Configures Sybil to run doc examples as tests.
docs/pymetkit/conf.py	Adds Sphinx configuration (autoapi, theme, exclusions).
docs/pymetkit/build_docs.sh	Adds a script to build the Sphinx documentation.
docs/pymetkit/bindings.rst	Documents the low-level pybind11 binding surface.
docs/pymetkit/api.rst	Documents the Pythonic API via sphinx-autoapi.
CMakeLists.txt	Adds a new PYTHON_METKIT_INTERFACE feature option.
cmake/pymetkit_setup.py.in	Adds setup.py template for wheel builds from the staged layout.
cmake/pymetkit_setup.cfg.in	Adds setup.cfg template (metadata/extras/python_requires).
.github/workflows/pymetkit.yml	Adds a CI workflow to build/test wheels across Python versions.
.github/workflows/cd.yml	Removes the wheel-python CD job invocation.

Comments suppressed due to low confidence (1)

tests/pymetkit/integration/mars_request/test_parse.py:104

• The test opens a file handle without closing it (parse_mars_request(open(...))). Use a context manager so the handle is reliably closed on all platforms.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.96%. Comparing base (2d8a915) to head (b87fe02).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #239   +/-   ##
========================================
  Coverage    50.96%   50.96%           
========================================
  Files          384      384           
  Lines        15461    15461           
  Branches      1309     1309           
========================================
  Hits          7879     7879           
  Misses        7582     7582           

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