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.
The pylint configuration in .pylint was a modified version of the
output of `pylint --generate-rcfile` from an unknown version of
pylint. Replace it with a file that only contains settings that are
modified from the default, with an explanation of why each setting is
modified.
The new .pylintrc was written from scratch, based on the output of
pylint on the current version of the files and on a judgement of what
to silence generically, what to silence on a case-by-case basis and
what to fix.
