Adds stubs for reportlab

[Daverball]

Large parts of this are still baseline stubs, but the higher level platypus API and some of the lib/pdfgen components it depends on are fairly complete and tested against several medium to large codebases of mine.

Docs can be found here: https://docs.reportlab.com/
There's currently no public git repository available for reportlab, so you'll have to install it and get the source that way.
It is still open source under the BSD license, so no worries there.

[srittau]

Due to the size, I've just scanned through it, and didn't compare with the implementation. The notes in stubtest allowlist make sense to me. If it turns out to be too restrictive, we can always loosen our annotations on request.

A few nits below, though.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉