Support PEP 561 (Distributing and Packaging Type Information)

[rchen152]

Type information for third party libraries will start being distributed outside of typeshed per PEP 561; we should support this.

[rchen152]

A specific part of this PEP that would be useful for us is in-project pyi files. They would allow, e.g., pytype users to indicate the existence of Cython modules.

[dbarnett]

Has any work started on PEP 561 support here?