auth: Use modern and more pythonic GSSAPI Python module

The kerberos package has been ported to Python 3 but has some quirks,
in particular taking regular strings where GSSAPI uses bytestrings.
3 jobs for gssapi
Status Job ID Name Coverage
  Test
failed #19756
run-test

00:03:13

failed #19754
run-test

 
  Build
skipped #19755
run-build
 
Name Stage Failure
failed
run-test Test
************* Module nbxmpp.auth
nbxmpp/auth.py:32:0: W0611: Unused GSSAPIState imported from nbxmpp.const (unused-import)

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Your code has been rated at 10.00/10 (previous run: 10.00/10, -0.00)

Running after_script
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ERROR: Job failed: exit status 1