Replace 'kerberos' pip dependency with 'gssapi'

[svmhdvn]

I'm currently working on an open source IT infrastructure project and
was trying to deploy a particular WSGI application with kerberos
support. I came across your project and would ideally like to use it in
the infrastructure. However, I'd just like to make some minor
improvements to it that should ideally benefit everyone.

Here are my proposals:

• Change project documentation to reflect that this is a SPNEGO (GSSAPI)
  middleware layer that supports Kerberos as the authentication backend
• Change the dependency on the relatively unmaintained and narrow python-kerberos[1] to
  the more general and well maintained gssapi[2].

I have the bandwidth to work on these changes now. I'd like to know if
these are reasonable proposals so that I can spend some time to submit a
PR to the project. Is that ok?

Thanks for the great project!

[propertone]

Sounds ok in theory, but I'm not familiar with the kerberos vs gssapi python library difference. @mkomitee @nrhall - do you know more about this?

[nrhall]

These seem like fine changes to me - I agree that the python GSSAPI module is the right way to go, looks like the author has now made the pykerberos repo read-only, and strictly this is indeed SPNEGO and not native kerberos...

[nrhall]

Happy to review as and when you have something, thanks!

[propertone]

Sounds good to me too then!

[svmhdvn]

Any feedback on #35?