build-sys: add clean-python target

build-sys: add clean-python target

Building for a different version of Python requires removing all
build products for the old version. There's no nice way to do it,
short of doing 'make clean'. The new 'clean-python' target is a
bit hacky, but seems to work:
  ./configure PYTHON=python2 && make && make install
  make clean-python
  ./configure PYTHON=python3 --disable-gtk-doc --disable-man-pages && make && make install
should install modules for both versions of Python.