Make check-python-files.sh run pylint on all *.py files (in
directories where they are known to be present), rather than list
files explicitly.
Fix a bug whereby the return status of check-python-files.sh was only
based on the last file passing, i.e. errors in other files were
effectively ignored.
Make check-python-files.sh run pylint unconditionally. Since pylint3
is not critical, make all.sh to skip running check-python-files.sh if
pylint3 is not available.