tests: add flake8 check This test will hopefully help prevent style errors. To be determined: which errors / warnings to ignore?

flake8: add file for rules to ignore This will help prevent users that have an editor with flake8 capabilities from fixing linter errors for rules that are not wanted by the project. Initially, this ignore list includes everything in the project that violates pep8. I'll try to ween this list down to something a little more manageable.

init: guard _namemap with repo.topics (issue5351) This prevents reading extra data on all repo commits when there is no topic for a given name. _namemap is called a lot, and it is often called for commit shas -- which should pretty much never be names of topics...