Hello,

I'm updating the Debian Package for mypy and very happy to have mypyc included.

When using the sdist from PyPI we are unable to build with MYPY_USE_MYPYC=1 as the mypyc/lib-rt directory is not present.

Switching to the autogenerated source code tarballs from the GitHub releases doesn't work because they lack the Git submodules, specifically mypy/typeshed

https://github.com/python/mypy/releases/tag/v0.730

We'd love to include the much faster and compiled version of mypy in Debian.

Options:

1. Tarballs that contain mypy/typeshed are added to the GitHub releases (manually at first, and then automatically)
2. MANIFEST.in is augmented to include the missing paths, such at mypyc/lib-rt and misc/proper_plugin.py (referenced from mypy_self_check.ini which we run)

mypyc/lib-rt/ is about 29K gzip'd, so I don't think that is too much to add to the sdist.

I'm happy to send a PR for option 2, just let me know.

Thanks, as always, for a fantastic project!