build,python: Fix uninstalling in distcheck

* lang/python/Makefile.am (uninstall-local): Get platlib path relative
to data path.

For unknown reasons (at least on my system) Python 3.1x's sysconfig
returns a platlib (== directory for site-specific, platform-specific
files) starting with /usr/local although the prefix config variable is
/usr. This broke uninstallation because a wrong folder was (tried to)
remove. Python 2.7 and 3.9 return a platlib starting with just /usr. In
order to always get the correct relative path we use the data path
instead of the prefix config variable as anchor directory.