Improve suffix parsing in issue_pattern

[dorschw]

Description

Fixes #653

Use internal issue name parser instead of parsing (again) the file name.

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.

[adiroiban]

Thanks. Looks good.