Base package pyrogen (for mypy)

[ChristianToepfer]

The cythonize package option, used in Better packaging #114, loses Proto-File namespace. 100 messages and more in one big file is not practical. 👎 To hold file structure helped. A base package over all generated pyrobuf message, would be great. I think that's equal to package scope #107 ?

MYPY support

On the other side a base package scope is needed to support static type checking, the other big advantage of cython generated code compared to google #136. All you need is to install a stub-only package pyrogen-stubs to check pyrobuf generated code in package pyrogen