Enable bugbear, fix warnings

pymbolic/compiler.py

pymbolic/geometric_algebra/__init__.py

pymbolic/geometric_algebra/mapper.py

pymbolic/mapper/__init__.py

pymbolic/mapper/c_code.py

pymbolic/mapper/collector.py

pymbolic/mapper/distributor.py

pymbolic/mapper/evaluator.py

pymbolic/mapper/substitutor.py

pymbolic/mapper/unifier.py

setup.cfg

test/test_pymbolic.py