[LINTER] Add mypy to the CI dependencies and exclude the contrib dir

Summary:
This will unbreak arc lint --everything and the land bot. Contrib
subfolders can be fixed and included in later diffs.
Also added a exclusion for sys.getwindowsversion() which is only
available on windows.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9302