Add flake8-bugbear as additional flake8 plugin

[hendrikmakait]

flake8-bugbear (https://github.com/PyCQA/flake8-bugbear) is a plugin for flake8 that contains additional rules aimed at finding likely bugs and design problems in the code. Notable rules include:

• B006: Do not use mutable data structures for argument defaults.
• B018: Found useless expression.

At the time of this writing, adding flake8-bugbear spits out 166 problems, most of which are in tests. If we decide to go with flake8-bugbear, we could ignore it for tests and clean those up over time before flipping the switch.

[fjetter]

+1 I love automated code reviews