[TODO] Improve towncrier check to support postfix number and extra suffices

[webknjaz]

• Add explicit check for pypy venv tox-dev/tox#1356 (comment)
• Expose the file name check regex as a public API twisted/towncrier#153

Valid fragment examples:

• docs/changelog/smth-else.deprecation (non-strict, not bound to an issue or a PR)
• docs/changelog/1355.misc (bound to an issue, it's implicit number is 0)
• docs/changelog/1355.bugfix.rst (bound to an issue, it's implicit number is 0, has a postfix and a suffix)
• docs/changelog/1355.bugfix.1.rst (bound to an issue, it's explicit number is 1, has a postfix and a suffix)
• docs/changelog/1355.feature.0.rst (bound to an issue, it's explicit number is 0, has a postfix and a suffix)
• docs/changelog/1355.feature.1.rst (bound to an issue, it's explicit number is 1, has a postfix and a suffix)