Check news entries suffix

[gforcada]

Now that more and more Plone packages are using towncrier, changelog entries are written on the news folder.

mr.roboto already checks for that with the changelog verifier, either if CHANGES.rst is modified or one of the modified paths has news/ on it (see f44cc38#diff-c0cc17c08d536788fe1179a3958aed52 )

This check for news entries though can be further improved: plone.app.upgrade could not be released as the news entry there was not valid (see plone/plone.app.upgrade@14bf8c5 )

mr.roboto should check that the suffix of the news entry file is valid, according to the pyproject.toml file.