Factor out type check in Linter actions

[sonniki]

We want to factor out the idiom we use in our Linter scripts to check the type of files that they receive as input. It looks like this, for example, in amp_add_class_frames:

if not liutils.is_py_file(file_name):
    # Apply only to Python files.
    _LOG.debug("Skipping file_name='%s'", file_name)
    return []

We want to make it a method in the parent class of all Linter classes, Action. And then we should go over all the scripts in linters and replace the idiom with the new method everywhere applicable.