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)

Author: svmhdvn

README.rst

@@ -4,7 +4,7 @@ WSGI-Kerberos
 WSGI-Kerberos is `WSGI`_ Middleware which implements `Kerberos`_ authentication.
 It makes it easy to add Kerberos authentication to any WSGI application.
 
-Its only dependency is `python-kerberos`_ and it's been tested up to version 1.3.0
+Its only dependency is `gssapi`_ and it's been tested with v1.8.3 onwards.
 
 You can install the requirements from PyPI with ``easy_install`` or ``pip`` or
 download them by hand.

example/example_application.py

@@ -1,4 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
+
 import logging
 from wsgi_kerberos import KerberosAuthMiddleware
 from wsgiref.simple_server import make_server

requirements.txt

@@ -1 +1 @@
-kerberos
+gssapi>=1.8.3

setup.py

@@ -36,14 +36,13 @@
       include_package_data=True,
       package_data={'': ['LICENSE', 'AUTHORS']},
       platforms='any',
-      install_requires=['kerberos'],
+      install_requires=['gssapi'],
       classifiers=['Development Status :: 4 - Beta',
                    'Environment :: Web Environment',
                    'Intended Audience :: Developers',
                    'License :: OSI Approved :: BSD License',
                    'Operating System :: OS Independent',
                    'Programming Language :: Python',
-                   'Programming Language :: Python :: 2.7',
                    'Programming Language :: Python :: 3',
                    'Topic :: Internet :: WWW/HTTP',
                    'Topic :: Internet :: WWW/HTTP :: WSGI',