`py.typed` not included in package build

[hf-kklein]

Relates to #191 where the py.typed file has been introduced.

Symptom

when using airesponses mypy complains:

error: Cannot find implementation or library stub for module named "aioresponses" [import]

Reason

Although the py.typed file has been added, the file alone is not enough to make this pacakge PEP561 compatible. The mypy docs state:

packages that supply type information via type comments or annotations in the code should put a py.typed file in their package directory.

This has been done in #193 ✅

But the mypy documentation also says:

you may also need to modify the inclusion rules in your MANIFEST.in

How to reproduce

• either use aioresponses somewhere: the error message from above comes up when running mypy
• or run python setup.py build: the py.typed file is not included in the build directory

[hf-kklein]

I tried to fix it in #207. When running python setup.py build locally, the py.typed file is included.