Status update week 8 GSoC

I finished up the new PythonCodePrinter in #12808. And I have started work locally on having the NumPy- and Mpmath-printers subclass this new CodePrinter. Both of these printers were previously subclassing LambdaPrinter, so the change is a bit intrusive with lots of failing tests in the SymPy test suite. But this is a prerequisite for changing sympy.utilities.lambdify to accept custom CodePrinter subclasses as user input.