patchtest: make pylint tests compatible with 3.x

pylint 3.x has removed epylint, which is now a separate module. To avoid
adding another recipe or using outdated modules, modify the
test_python_pylint tests so that they use the standard pylint API.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>