Edit-extension

[SmileyChris]

Description

The temporary file towncrier create creates now uses the correct .rst or .md extension, which may help your editor with with syntax highlighting.

Fixes #527

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. 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).

[adiroiban]

Looks good. Thanks.

[adiroiban]

Many thanks Chris for the improvements!