spec file: rename the python-polib dependency name to python2-polib
Trying to install the package depending on python-polib breaks
when the system has newer (and renamed) version python2-polib.
*This patch is an workaround* for the issue described in [1].
If a renamed package's provides is equal to an older package's
name, dnf tries to install the older package.
When the newer package is in the system, this leads to a conflict.
[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1096506
Reviewed-By: Martin Basti <mbasti@redhat.com>