[lint] run mypy on contrib

Summary:
In most cases, the errors are fixed by simply adding a typehint. In one case, initializing a variable with 0. (float) instead of 0 (int) is necessary. In another case, I had to check that a variable really is a list before accessing one of its value by index.

In the case of raise UnicodeDecodeError, mypy found an actual bug.

>>> raise UnicodeDecodeError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: function takes exactly 5 arguments (0 given)

Rather than figuring out what the 5 arguments should be, it is simpler (and cleaner) to raise and catch a custom error for that particular case.

I skipped contrib/macdeploy, because this module has some hairy errors. It can always be linted in a separate diff in the future.

Depends on D13637

Test Plan: arc lint --everything

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13638