python: uninstall cater to PEP-3147

python: uninstall cater to PEP-3147

After recent commit commit v1.12.4-43-ge0e99ed, "python: improve
support for modern python (CPython 3.2 and PyPy)", the python install
rules have been made smart enough to install "*.pyc" byte-compiled
files according to PEP-3147 with modern (post-3.2) pythons.  However,
the uninstall rules hadn't been updated accordingly, causing leftover
files to remain around after "make uninstall", as well as failures in
"make distcheck".

* lib/am/python.am (am__pep3147_tweak): New internal macro, used
in ...
(uninstall-%DIR%PYTHON): ... the recipe of this target, which has
been adjusted to cater to PEP-3147

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>