Add `.gitkeep` as one of default valid filenames

[stevepiercy]

With the change from #622, a very common filename .gitkeep was omitted from the list of valid filenames.

Would you please consider adding the filename .gitkeep to the list at https://github.com/MetRonnie/towncrier/blob/9b685033b22bd3ee1bd81955a333e0e20bef02e6/src/towncrier/_builder.py#L117?

I would be happy to submit a PR to save myself several hours of work to modify our repositories in the Plone organization alone.

Thank you for your consideration.

Ref plone/volto#6225

[adiroiban]

Thanks for report.

Yes. I would be happy to have it to the list.

[stevepiercy]

@adiroiban PR added: #647. Thank you!