ci: Test httpx and httpx2 side by side

[cclauss]

Let's test Pydantic's httpx2 side by side with the original httpx.

Related to:

• httpxyz/httpx2 support #57
• https://pydantic.dev/docs/httpx2
• https://pydantic.dev/docs/httpx2/get-started/migration

Test results: https://github.com/cclauss/httpx-gssapi/actions/runs/35281657849

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pip install -U can be quite opaque to readers because pip install currently has six --u* options, including --user.

pip install --upgrade makes the intention clearer in code that others will read.

% python3 -m pip install --help | grep "\-\-u"

  --user                      Install to the Python user install directory for
  -U, --upgrade               Upgrade all specified packages to the newest
  --upgrade-strategy <upgrade_strategy>
  --use-pep517                Use PEP 517 for building source distributions
  --use-feature <feature>     Enable new functionality, that may be backward
  --use-deprecated <feature>  Enable deprecated functionality, that will be