remove all usage of six

pymbolic/algorithm.py

pymbolic/compiler.py

pymbolic/cse.py

pymbolic/geometric_algebra/__init__.py

pymbolic/geometric_algebra/mapper.py

pymbolic/imperative/statement.py

pymbolic/imperative/utils.py

pymbolic/interop/maxima.py

pymbolic/mapper/__init__.py

pymbolic/mapper/coefficient.py

pymbolic/mapper/collector.py

pymbolic/mapper/constant_folder.py

pymbolic/mapper/evaluator.py

pymbolic/mapper/unifier.py

pymbolic/parser.py

pymbolic/polynomial.py

pymbolic/primitives.py

pymbolic/rational.py

pymbolic/traits.py