The BFSS API changes in python 3:

* 'defaultdir' MUST be a unicode object
* 'fsencoding' MUST NOT be set

In python 2, fsencoding handles both the encoding of file paths on the
file system (utf-8 by default, but the system may actually be using
something else) and the encoding of file paths that will be stored in
the database.

So in python 3, we wipe the slate clean:

* rely on sys.getfilesystemencoding() to convert unicode objects to
  bytes
* always encode paths to utf-8 for storage in the database

Caveat emptor / here be dragons:

* sys.getfilesystemencoding() depends on the current locale, which
  therefore MUST be set properly
* when migrating an existing instance from py2 to py3, one MAY need
  to reencode file paths stored in the database