Add regression tests for redis/requests-cache version compatibility

[iandees]

Summary

Regression test for the redis-py/Redis-server version incompatibility that took down production on 2026-07-28 (fixed in #98).

census_extractomatic/test_redis_compat.py spins up a real redis:5.0.7 container (not a mock — required to reproduce the HELLO/protocol-version behavior), configures requirepass, connects via a user:pass@host URL matching production's shape, and exercises the app's real flask_caching.Cache -> cachelib -> redis-py path. Plus a guard test asserting redis stays explicitly pinned in Pipfile.

Verified catches the bug: passes with redis==5.1.1; fails with redis==8.0.1, reproducing the exact production traceback (redis.exceptions.ResponseError: unknown command 'HELLO'...).

Also added: a GitHub Actions workflow (.github/workflows/test.yml) to actually run this on push/PR — neither repo had any CI before this. It installs the locked dependencies (minus gdal, which needs a matching system libgdal the runner doesn't have — see note below) and runs the full test suite. Confirmed passing on a real GitHub Actions run.

Known gap, not addressed here: this CI doesn't cover GDAL-dependent code (census_extractomatic/api.py's full import chain, via user_geo.py's from osgeo import ogr), since that needs the system libgdal to match the pinned Python gdal package version exactly (see the Dockerfile/Pipfile comments from the GDAL sync fix). None of the current tests touch that code path, so it wasn't blocking, but it's worth its own follow-up if test coverage expands into that area.

Test plan

• pytest census_extractomatic/test_aggregate_acs.py census_extractomatic/test_moe.py census_extractomatic/test_redis_compat.py (22/22 passing locally)
• Confirmed the new test fails against redis==8.0.1 and passes against redis==5.1.1
• GitHub Actions workflow passing on a real run: https://github.com/censusreporter/census-api/actions/runs/30395519426