Enforce fragment name structure

[dorschw]

Description

Fixes #649

Add a config for enforcing the fragment filename stems, using regex.

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure test pass on your local environment.
• Create a file in src/towncrier/newsfragments/. Briefly describe your
  changes, with information useful to end users. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.
• If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[adiroiban]

Thanks. Changes looks good.

We need all the tests to pass in order to merge this.

[dorschw]

Thanks @adiroiban, any idea why it fails with Linux PyPy/CPython 3.8?
Using linux and Python 3.8, all tests pass locally using trial.

[adiroiban]

I don't know why it fails. Do you have 3.8.18 on your local system ?

[dorschw]

got it (it was removesuffix, probably some env mess on my machine made it run on non-3.8), please trigger CI again and it should be good to go.

[adiroiban]

Looks good. Thanks