remove python2 shebang in hash module

[lukpueh]

Description of issue or feature request:
securesystemslib/hash.py has a python2 shebang line, which is obsolete (Py2 support was dropped a while ago) and wrong (no need run the module by itself).

Current behavior:

securesystemslib/securesystemslib/hash.py

Line 1 in 416dab2

#!/usr/bin/env python2

Expected behavior:
No shebang line.

Bonus task:

Review eligibility of shebang lines in other modules too.