COMPAT: Properly encode filenames in read_csv
Python 3.6+ changes the default encoding to `utf8` ([PEP 529](https://www.python.org/dev/peps/pep-0529/)), which conflicts with the encoding of Windows (`mbcs`).  This conflicts rears its head when using only the C engine of `read_csv` because of the low level functionality that we implement ourselves.

This fix checks if we're using Python 3.6+ and on Windows, after which we force the encoding to `mbcs`

Closes #15086.