Collection of libcst fixers to aid with the removal of dependencies on
the library "six" from code that was once straddling py2/py3 and now only
needs py3 compatibility.

It is expected that we can mostly automate most changes except for
six.ensure_str, which is ambiguous (either a text->text no-op, a
binary->text decode, or a conditional decode that is accepting both
binary and text). It may be possible later to involve mypy to aid in
this.

For six.ensure_binary and six.ensure_text, we are assuming that the
original datatype was 'str', so six.ensure_text will resolve to a no-op
and six.ensure_binary will resolve to an encode. (Reasoning: if it was
not a 'str', then either encode/decode could have been used, and cases
where there was a union of either should not have been using six.)