Making Kerberos optional in the Python wheel

There have been a couple of complaints that the latest Python wheel
distribution doesn't install cleanly. This happens when there is no
pre-built gssapi wheel on PyPi that matches the users combination of Python
version and platform, *and* the MIT Kerberos development headers etc. are
not present on the system, so the source wheel cannot be compiled.

This seems like it's a bit onerous on users, especially if they're on
Windows where they'll also need a suitable compiler to be installed. The
attached patch aims to address that by making the Kerberos support optional
(thankfully, Khushboo made the code handle lack of gssapi libraries).

To install without gssapi, users would simply do:

pip install pgadmin4

or

pip install pip install /path/to/pgadmin4-5.0-py3-none-any.whl

To install with gssapi:

pip install pgadmin4['kerberos']

or

pip install pip install /path/to/pgadmin4-5.0-py3-none-any.whl['kerberos']

The patch also cleans up some old cruft that was required for now
unsupported Python versions.

Thoughts?

--
Dave Page
Blog: http://pgsnake.blogspot.com
Twitter: @pgsnake

EDB: http://www.enterprisedb.com