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
9 jobs for shebang in 2 minutes and 20 seconds
latest
Status Job ID Name Coverage
  Test
passed #110592806
docs

00:01:00

passed #110592807
dogfood

00:00:42

passed #110592805
linters

00:01:35

passed #110592800
python2

00:00:51

passed #110592801
python34

00:00:51

passed #110592802
python35

00:01:00

passed #110592803
python36

00:00:47

passed #110592804
python37

00:01:15

 
  Build
passed #110592808
build

00:00:44