Never use an existing enum module. (#168)

* Never use an existing enum module.

* Add comment.

Author: Thom1729

st3/sublime_lib/_compat/enum.py

@@ -1,4 +1,4 @@
-try:
+if False:  # For MyPy only
     from enum import *  # noqa: F401, F403
-except ImportError:
+else:
     from ..vendor.python.enum import *  # type: ignore # noqa: F401, F403