Ignore invalid calendar X-WR-TIMEZONE

[RasputinKaiser]

Fixes #31

Summary

• Treat invalid X-WR-TIMEZONE values from the input calendar as absent instead of raising during conversion.
• Preserve strict behavior for an explicitly passed invalid timezone= argument.
• Add regression coverage for the invalid calendar field and explicit-argument cases.

Duplicate check

• Checked existing PRs for issue Invalid X-WR-TIMEZONE should not fail #31, Invalid X-WR-TIMEZONE, and d6708488efb330c0; no matching PRs found.
• Checked issue Invalid X-WR-TIMEZONE should not fail #31 comments before submitting; no visible competing fix/discussion was present.

Tests

• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python -m pytest test/test_convert_examples.py test/test_copy.py test/test_command_line.py -q
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python -m pytest -q
• sh test/test_code_quality.sh
• git diff --check

Note: CONTRIBUTING.md asks for changelog entries except trivial changes, but I could not find a changelog/news file in this repository to update.

[niccokunzmann]

Thank you for your contributions! Would you like to co-maintain this project?

[niccokunzmann]

Thanks for this PR!

I think, we have calendar files for the tests. Please put this in there to make sure that the other tests pick them up.