dCache should migrate all Python v2 scripts to use Python v3

[paulmillar]

As has been reported in #5956, Python v2 is deprecated and distributions are starting to drop their support for this version of Python.

• There are some support scripts within dCache code-base that use python. These should be updated to use Python v3 (usually requiring only minimal changes).
• The packaging files (RPM's .spec files and Debian's equivalent) should be updated to reflect that there is no Python v2 dependencies; for example, by updating Python dependencies to versions that support Python v3.

Note that the preferred hash-bang for Python 3 is:

#!/usr/bin/env python3