Run all checks on a Python source file.

def input_file(filename):
    """
    Run all checks on a Python source file.
    """
    if excluded(filename) or not filename_match(filename):
        return {}
    if options.verbose:
        message('checking ' + filename)
    options.counters['files'] = options.counters.get('files', 0) + 1
    errors = Checker(filename).check_all()
    if options.testsuite and not errors:
        message("%s: %s" % (filename, "no errors found"))
    return errors