Make it easier to check the formatting of a newsfragment locally

[jakkdl]

Currently if you want to check the validity of any newsfragment locally you need to do something like

export READTHEDOCS=1
cd docs
make html

but then towncrier will attempt to remove your newsfragments as well as update changelog.rst, which means you need to afterwards restore/unstage the changes to those files.

It would be great to have an invocation of a shell script/tox/hatch/make target/etc that checked the formatting of the fragments without messing up local files.

[TeamSpen210]

Hmm, currently towncrier only runs if READTHEDOCS is set. What we could do instead is use --keep so it doesn't remove the newsfragments - we only really want that when committing a release. Have the sphinx conf.py first read in the changelog file, run towncrier, then revert the file after the build finished.

[jakkdl]

Reopening in case we want to change the implementation, see discussion in the linked PR.