Fix Python 3.14 compatiblity

[thearpankumar]

❓ What kind of change does this PR introduce?

• [x] 🐞 bug fix
• [ ] 🐣 feature
• [ ] 📋 docs update
• [ ] 📋 tests/coverage improvement
• [ ] 📋 refactoring
• [ ] 💥 other

📋 What is the related issue number (starting with #)

Resolves #767

❓ What is the current behavior? (You can also link to an open issue here)

Python 3.14 tests fail in CI with:

• Multiprocessing errors due to default start method change from fork to forkserver on Unix
• IOBase cleanup errors during test teardown due to changed error message format
• Pre-commit failures from flake8-annotations incompatibility

❓ What is the new behavior (if this is a feature change)?

All tests pass on Python 3.14 (Linux and Windows) with full backward compatibility for Python 3.8-3.13.

📋 Other information:

Root Cause: Python 3.14 changed the default multiprocessing start method from fork to forkserver on Unix platforms, breaking pytest-xdist parallel test execution.

Solution Applied:

1. Multiprocessing Fix (cheroot/test/conftest.py):

   • Force fork method on Python 3.14+ Unix systems only
   • Skip on Windows (doesn't support fork)
   • Clean exception handling with contextlib.suppress

2. IOBase Warning Filter (pytest.ini):

   • Updated regex pattern to match Python 3.14's new error format
   • Changed from "Exception ignored in." to "Exception ignored.*"

3. Pre-commit Config (.pre-commit-config.yaml):

   • Set flake8 language_version: python3.11 to match CI
   • Prevents flake8-annotations Python 3.14 incompatibility

Testing: Verified on Python 3.14.0rc3 with pytest-xdist parallel execution (n=auto).

📋 Contribution checklist:

• [x] I wrote descriptive pull request text above
• [x] I think the code is well written
• [x] I wrote good commit messages
• [ ] I have squashed related commits together after the changes have been approved
• [x] Unit tests for the changes exist
• [x] Integration tests for the changes exist (if applicable)
• [x] I used the same coding conventions as the rest of the project
• [x] The new code doesn't generate linter offenses
• [x] Documentation reflects the changes
• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

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