Update Python version support and fix PyPy test environments

[Perlence]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added test(s) covering the changes (if testable)

Python Version Updates

• Added Python 3.14 to the stable test matrix.
• Added PyPy 3.11 to the test matrix.
• Updated the unstable matrix from 3.14-dev to 3.15-dev.
• Removed Python 3.9, it wasn't used in Actions.

PyPy Build Fix

Recent versions (1.19.0+) of mypy depend on librt. Unfortunately, mypy is included in pyproject's build system requirements, which are installed in each tox test environment. Specifically, librt fails to compile on PyPy, which breaks PyPy tox environments. Ironically, PyPy builds don't even need mypyc; only CPython release builds use it.

One option is to pin mypy in build-system.requires to mypy<1.19. However, this way, we'll miss out on new advancements in mypyc.

I opted for another option: skipping build isolation and installing build requirements manually in PyPy test environments.

Issue(s) Resolved

Fixes #

Affirmation

By submitting this Pull Request or typing my (user)name below, I affirm the Developer Certificate of Origin with respect to all commits and content included in this PR, and understand I am releasing the same under openskill.py's MIT license.

I certify the above statement is true and correct: Perlence

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cdaa9b3) to head (2ef27fd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #184   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1972      1972           
  Branches       488       488           
=========================================
  Hits          1972      1972           

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[vivekjoshy]

@all-contributors please add @Perlence for code

[allcontributors]

@vivekjoshy

I've put up a pull request to add @Perlence! 🎉