Drop PyPy from the CI test matrix

[syssi]

Problem

The pypy3.9 CI job fails because cryptography>=49 uses pyo3-ffi v0.29.0 which requires PyPy >= 3.11. There is no version of PyPy that satisfies all dependency constraints simultaneously:

• cryptography has no prebuilt wheels for PyPy 3.11+ and source builds require pyo3 >= 0.23
• pydantic-core dropped prebuilt PyPy 3.10 wheels in recent versions
• Older cryptography versions (43.x) cap out at PyPy 3.10 via pyo3 0.22.2

Fix

Remove PyPy from the CI matrix. The Rust-based dependency ecosystem (cryptography, pydantic-core) no longer provides a stable PyPy target that works across all supported versions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.26%. Comparing base (507afce) to head (f6887c2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2073   +/-   ##
=======================================
  Coverage   82.26%   82.26%           
=======================================
  Files         197      197           
  Lines       19145    19145           
  Branches     1052     1052           
=======================================
  Hits        15750    15750           
  Misses       3218     3218           
  Partials      177      177           

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[syssi]

@rytilahti Could you remove the job (Python pypy3.9 on ubuntu-latest) from the branch protection rules? Thanks in advance!

[rytilahti]

@syssi done! thanks for working on getting things cleaned up! Python 3.9 support should be dropped completely as it is end-of-life already (https://devguide.python.org/versions/), I would say that (at least) 3.10 can also go as its support ends later this year.

We can take a look at the usage data to see which versions are used by the people downloading the current release (looks like it is still being actively downloaded :o), and maybe consider modernizing our target to be even newer than that.

[syssi]

Let's drop the end-of-life python versions in a dedicated pull request after the official 0.6.0 release.