fix: Source freshness errors with optional filter arg

[eliyarson]

Description

When using optional freshness argument filter the hook check_source_has_freshness.py returns the following error:

AttributeError: 'str' object has no attribute 'keys'

It happens because the hook is expecting dictionaries (warn_after and error_after), but filter is a string.
A simple fix is to pop the key if it exists before checking for required keys as I did in the PR.

Obs: I had to change github flake8 pre-commit hook from gitlab to github because I don't have a gitlab acc, feel free to change back to gitlab if you guys think it's necessary.

Testing

pytest tests/unit/test_check_source_has_freshness.py