Remove unused files from wheel files

[dosisod]

I was poking around the wheel files for this project, specifically this one, and found that there are a lot of files we probably don't need in there.

Among the files/folders are:

• mypy/xml/
• mypyc/external/googletest/
• mypyc/doc/

Do we need these files? Thanks

[hauntsaninja]

mypy/xml should be used at runtime by the report generation stuff and is needed. For the other two, probably fine to skip in the wheel, but we need them in the sdist because people get upset if the sdists don't contain tests and docs and stuff.