python: properly uninstall __pycache__ in subdirectories

python: properly uninstall __pycache__ in subdirectories

This change finishes fixing automake bug#32088:
https://debbugs.gnu.org/32088

* lib/am/python.am (uninstall-%DIR%PYTHON): Uninstall
byte-compiled files in a '__pycache__' subdirectory
from sub/__pycache__/, not __pycache__/sub/.
(am__pep3147_tweak): prefix __pycache__ here, for both .pyc and .pyo.
https://lists.gnu.org/archive/html/automake-patches/2018-05/msg00000.html
* t/instmany-python.sh (limit): increase to 4500, following
Andreas Huettel, https://debbugs.gnu.org/32088#11.