Add workaround for #31 (clash with six @ Python 3)

The "six" library changes the list of modules on the fly when we access
a module attribute; this causes a "RuntimeError: dictionary changed size
during iteration" under Python 3. This workaround excludes modules matching
`six.moves.*` from attribute checking on "unpatching" phase.
Merge pull request #32 from neithere/patch-1

Add workaround for clash with "six" on Python 3