Disable mercurial demandimport in _pyflakes module

Disabling in hooks is too late since mercurial already imports
_pyflakes before.

Fixes #88

flake8/hooks.py

@@ -6,12 +6,6 @@ import stat
 from subprocess import Popen, PIPE
 import shutil
 from tempfile import mkdtemp
-try:
-    # The 'demandimport' breaks pyflakes and flake8._pyflakes
-    from mercurial import demandimport
-    demandimport.disable()
-except ImportError:
-    pass
 try:
     from configparser import ConfigParser
 except ImportError:   # Python 2