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

• [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).

[adiroiban]

Many thanks Chris for the improvements!