We need this as we no longer use setuptools_scm to retrieve the full list of files under version control.
Instead, we need to spell them out one by one.
Hopefully we don't forget anyone...
One notable absence was README.md... :)
For the testdata/*, I now use `include_package_data=True` in setup.py, to automatically install any data files found inside package directories that are also specified in MANIFEST.in file.
I also include the Snippets, which I think should belong to the source distribution, despite not being installed as console scripts like ttx, etc.
The MANIFEST.in does not need to include itself (done by default).