Apply mypy typing checker to all files

[SagiPolaczek]

Is your feature request related to a problem? Please describe.
As described here, we use black, flake8 and mypy to enforce certain format criteria.
Currently we are ignoring some of the issues and some of files for the mypy checker. See ./.mypy.ini file for more details.

Describe the solution you'd like
Apply static typing in the ignored files so mypy will pass without ignoring any files (or at least much smaller amount).

NOTES

1. NO LOGIC SHOULD BE CHANGED. Only the static typing according the functions/classes properties.
2. This task might take too much time and effort in one-shot, so it can be done in a small doses - each PR with some amount of files fixed.

[SagiPolaczek]

Solved in #304