Potentially remove all those pesky .pyi files

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1412)
In GitLab by [[Gitlab user @tristanvb]](https://gitlab.com/tristanvb) on Dec 2, 2020, 10:07

Apparently, it is possible to [encode pep484 type information directly into cython modules](https://cython.readthedocs.io/en/latest/src/tutorial/pure.html#pep-484-type-annotations)

It could be nice to remove all the redundant forward declarations...

Needs investigation, possibly we need to use [a mypyc extension](https://github.com/python/mypy/tree/master/mypyc#mypyc-mypy-to-python-c-extension-compiler) for validation in CI.

Start by reviewing the referenced Cython PEP 484 documentation and locating the existing .pyi files and their CI validation. Determine whether type information can be moved into the Cython modules and whether mypyc is needed for validation. Done means redundant forward declarations are removed without losing type-checking coverage.

Written by the indexing model from the issue text.