Fix building wheels with py.typed files

[srittau]

No description provided.

[srittau]

Currently, the integration tests fail, since py.typed is not recognized. I'm not sure, why the tests didn't flag this in #109. Cc @Avasam

[srittau]

I think the tests fail locally, because previous runs create py.typed, which subsequent runs then pick up. I'll investigate more.

[Avasam]

I think we check for file extensions to only accept .pyi and .md found in typeshed.
But after running once, a py.typed is added, which is then seen on a second run because the run is not cleaned up locally (on CI it doesn't matter since it's a temporary clone of typeshed iirc)

Ironically I was manually cleaning up the extra folders and files locally between runs because I was testing the file/folder generation.

I see some potential solutions:

1. We could allow py.typed in our check. But that will allow py.typed files to slip in typeshed's source. We may not want that?
2. Add clean-up code. (probably a good idea to avoid further similar issues of polluting a local typeshed repo)
3. Change how/where the physical py.typed file is generated

[srittau]

The main problem here is that the integration tests modify files outside the stub uploader directory and don't clean up. I'll open a separate issue.