Mk/Uses/python.mk: Fix PLIST issue for USE_PYTHON=pep517

Currently "USE_PYTHON=autoplist pep517" generates the PLIST as follows:

• Extract the list of installed files from the RECORD file to the intermediate PLIST (_PYTHONPKGLIST)
• Manipulate the intermediate PLIST
• Add list of pycache files to the intermediate PLIST

When the RECORD file contains foo.pyc entry, that file will be counted twice in
the PLIST at the end. It will cause check-plist error. This fix removes *.pyc
entries while manipulating the intermediate PLIST to ensure all pycache files
are only counted once.