Replace PyKerberos dependency with python-gssapi

[svmhdvn]

Replace PyKerberos dependency with python-gssapi

This patch comprises the following changes:

• Replace 'kerberos' with 'gssapi'
• Remove lingering python2 support
• Use a real kerberos realm for running tests (i.e. remove mock calls)

[svmhdvn]

Please test this PR and let me know of any feedback/fixes. All existing tests are passing with a real kerberos realm instead of mock calls now.

[mkomitee]

I think in principle, we're on board with a change from the kerberos library to the gssapi library, but since you coupled that with a restructuring of the repository, it's quite difficult to review. Would you be willing to separate the changes related to the library replacement out into its own PR?

[svmhdvn]

For sure! I'll do that and submit a couple separated PRs soon.

[svmhdvn]

Updated the patch to only make the switch to the 'gssapi' python library. Let me know if you need clarification on any of the changes and I can explain in detail. I tried to make the diff as small as possible, but the test changes are still large since the testing methodology is changed in this patch.

[abompard]

This makes setup.py fail

[abompard]

Maybe default to socket.gethostname()?

Suggested change

	if hostname:
	if not hostname:
	hostname = socket.gethostname()