cast kernel cwd to bytes on Python 2 on Windows

Popen on Windows Python 2.x apparently cannot handle unicode cwd.
use filesystemencoding if defined, fallback on ascii.

closes #3039
Merge pull request #3046 from minrk/popen_cwdu_win32

cast kernel cwd to bytes on Python 2 on Windows

Popen on Windows Python 2.x apparently cannot handle unicode cwd. use
filesystemencoding if defined, fallback on ascii.

closes #3039