Add stubs for clingo python module.

[rkaminsk]

This pull request continues the work in #188 to add stubs for clingo's python module. The stubs are generated from the information in the docstrings and can be updated with the scratch/mypy.py script.

With this a python program using the clingo module can be type checked. For example running mypy on the program

import clingo

ctl = clingo.Control()

ctl.ground([("part", [10])])

in file test.py results in

$ mypy test.py
test.py:5: error: List item 0 has incompatible type "int"; expected "Symbol"
Found 1 error in 1 file (checked 1 source file)

because clingo.Number(10) should be used instead.

• Parse docstrings to generate stub files.
• Install stub files along with python module.
• A lot of annotations use List where just an Iterable is required. This has to be fixed to make annotations usable in practice.

There might be more issues regarding annotations because it is the first time that they are really used but this is probably best fixed in future commits when type checking real projects.