* fix PEP8 E721
do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
* remove redundant parentheses
* fix SiteWorker.run for empty job queue
local variable job is not assigned if queue was empty
when calling .run(), but it is used in exception handler.
* remove unreachable code in parse_diff_lines
* bug fix for unreachable code in is_worker_running
the code intended to check if *any* worker is running for
any site was *unreachable*.
this caused false negative results for site=None.
* check_failed_response: remove outdated part of docstring
* pull request template: fix relative path to LICENSE.txt
* fix typos
* use logger.warning, .warn is deprecated
