Add shebang to identify dev_tools/pypath as bash script

Help shellcheck to recognize bash syntax in dev_tools/pypath.

Note the executable flag on `dev_tools/pypath` is deliberately off.
The file is intended for sourcing from other shell sessions.

Author: pavoljuhas

dev_tools/pypath

@@ -1,3 +1,5 @@
+#!/usr/bin/env bash
+
 # Copyright 2018 The Cirq Developers
 #
 # Licensed under the Apache License, Version 2.0 (the "License");