package/{pkg-generic, python, python3}: rename .py file exclusion variable to not conflict

Fixes:
http://autobuild.buildroot.net/results/3b6/3b6280b0b7a9634b747db2865b21c6266007c725/



The PYTHON_KEEP_PY_FILES global variable conflicts with the per-package
<pkg>_KEEP_PY_FILES variable for the python package, causing make to
complain:

package/zlib/zlib.mk:7: *** Recursive variable 'PYTHON_KEEP_PY_FILES' references itself (eventually).  Stop.

As a workaround, rename the global variable to KEEP_PYTHON_PY_FILES so it
cannot conflict with the per-package variable.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>