Lint Python files with a shebang

When a pattern is not passed to flake8 (--filename), look for all files
that end with .py as well as extension-less files that start with a
Python shebang. Helps project lint scripts that may not have an
extension.

Fixes #409

tests/fixtures/example-code/script

@@ -0,0 +1 @@
+#!/usr/bin/env python