py3: provide encoding.environ which is a dict of bytes This can't be moved to pycompat.py since we need encoding.tolocal() to build bytes dict from unicode os.environ.

py3: convert encoding name and mode to str Otherwise tolocal() and fromlocal() wouldn't work on Python 3. Still tolocal() can't make a valid localstr object because localstr inherits str, but it can return some object without raising exceptions. Since Py3 bytes() behaves much like bytearray() than str() of Py2, we can't simply do s/str/bytes/g. I have no good idea to handle str/bytes divergence.

pycompat: extract function that converts attribute or encoding name to str This will be used to convert encoding.encoding to a str acceptable by Python 3 functions. The source encoding is changed to "latin-1" because encoding.encoding can have arbitrary bytes. Since valid names should consist of ASCII characters, we don't care about the mapping of non-ASCII characters so long as invalid names are distinct from valid names.