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)
* Reformat files with 'black'

Future TODO:
* Modernize python packing with latest guidelines
* Support all GSSAPI backends generically and remove Kerberos references
* Test more situations

.gitignore

@@ -1,4 +1,5 @@
 *.py[cod]
+venv
 
 # C extensions
 *.so

requirements.txt

@@ -1 +1,2 @@
-kerberos
+gssapi
+k5test

setup.py

@@ -36,18 +36,17 @@
       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',
                    'Topic :: Internet :: WWW/HTTP :: WSGI :: Middleware',
                    'Topic :: Software Development :: Libraries :: Python Modules'],
       test_suite='test_wsgi_kerberos',
-      tests_require=['mock', 'WebTest'])
+      tests_require=['k5test', 'requests', 'requests-gssapi', 'wsgi-intercept'])