Enable reporting of encoding warnings.

[jaraco]

Description

As suggested in #561 and #577, enable reporting of EncodingWarnings per PEP 597.

This PR is proposed separately from #577, because #577 only fixes the warnings in the main codebase and not in the tests. 80-some warnings remain to be addressed, so I leave it to the maintainers to determine when to add this check.

Checklist

• [x] Make sure changes are covered by existing or new tests.
• [x] For at least one Python version, make sure local test run is green.
• [x] Create a file in src/towncrier/newsfragments/. Describe your change and include important information. Your change will be included in the public release notes.
• [x] Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• [x] Ensure docs/tutorial.rst is still up-to-date.
• [x] If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• [x] If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[adiroiban]

Thansk for the PR... sorry for missing it.

Feel free to ping me if a PR is not reviewed.

I sent invitation to you and @SmileyChris to towncrier contributors team... if a PR is approved, you can trigger the merge.

[adiroiban]

I have enabled auto-merge for this. I hope it will land soon. Thanks again!