Be more precise with flake8 filename matches

In upcoming patchsets I'll be introducing filenames that include
"keystone-" as part of the name. These filenames should not be checked
with flake8 because they do not contain Python code.

bp functional-testing

Change-Id: I8330f0dca0d6c1db601ddecd71c6c92cef260c99
Merge "Be more precise with flake8 filename matches"