Run Flake8 during CI

pymbolic/algorithm.py

pymbolic/cse.py

pymbolic/functions.py

pymbolic/interop/ast.py

pymbolic/interop/sympy.py

pymbolic/mapper/flattener.py

pymbolic/mapper/flop_counter.py

pymbolic/maxima.py

pymbolic/parser.py

pymbolic/sympy_interface.py

setup.cfg

test/simple.py