Add support for PyPy

[movermeyer]

It would be nice if this library also worked with PyPy.
It might not be that hard to get it working.

Known issues

• Py_UNICODE_IS_SURROGATE doesn't exist in PyPy
  • Easy to vendor
• _PyUnicode_Copy doesn't exist in PyPy
  • Read bpo-34454: Clean up datetime.fromisoformat surrogate handling python/cpython#8959 for context, then vendor?
• The way we monkeypatch the methods is very cPython specific
  • Figure out the proper PyPy way