py3: have bytes version of sys.executable
sys.executable on Python 3 returns unicodes and we want bytes. So this patch
adds a new pycompat.sysexecutable which returns bytes by encoding using
os.fsencode() since it is path variable.
py3: use python 3 compatible variables in hgext/fsmontor/__init__.py
Earlier this was left thinking that its part of pywatchman package.
This patch replaces variables os.sep, sys.platform and os.envrion with their
py3 compatible ones.