EncodingWarning in pgen

Relating to #3658, if PYTHONWARNDEFAULTENCODING is set, this library raises a warning at:

https://github.com/psf/black/blob/2fd9d8b339e1e2e1b93956c6d68b2b358b3fc29d/src/blib2to3/pgen2/pgen.py#L42

