Workaround: setup.py regen_pot_file didn't work with babel < 1.0

A babel bug prevents `input_dirs` to be parsed correctly with babel pre-1.0 versions.
Using regen_pot_file with babel 0.9.6 leads to an empty pot file (but header).
The fix appears in babel 1.0 Changelog:
- fix ‘input_dirs’ option for setuptools integration (ticket #232, initial patch by Étienne Bersac)

The workaround just splits `input_dirs` as it should when old babel versions are used.