remove python2 shebang in hash module
**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securesystemslib/hash.py has a python2 shebang line, which is obsolete (Py2 support was dropped a while ago) and wrong (no need run the module by itself).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**:
https://github.com/secure-systems-lab/securesystemslib/blob/416dab215bdb0ae1f7d9b4c7ec9da8013100c069/securesystemslib/hash.py#L1

**Expected behavior**:
No shebang line. 

**Bonus task**:

Review eligibility of shebang lines in other modules too. 


diff --git a/securesystemslib/hash.py b/securesystemslib/hash.py
index 4ea04e1..3a3c2f4 100755
--- a/securesystemslib/hash.py
+++ b/securesystemslib/hash.py
@@ -1,4 +1,3 @@
-#!/usr/bin/env python2
 """
 <Program Name>
   hash.py
['securesystemslib/hash.py']