Make Sphinx doctree cache invalidation also depend on the Jinja 2 template changes

[webknjaz]

Currently, changes to the file specified in [tool.towncrier].template, in pyproject.toml or towncrier.toml don't cause rebuilding the document where our directive is used. I mean, updating any of the template file or the config file doesn't invalidate the doctrees cache, at the moment.
This problem existed for the change fragments too, in the past. And have been addressed via #1, #8 and #23.

Now, the same has to be done for the template too.