Update Trove classifiers

Drop python2, note our stability, and update python3 minor versions.

Author: frozencemetery

setup.py

@@ -305,11 +305,12 @@ def gssapi_modules(lst):
     license='LICENSE.txt',
     url="https://github.com/pythongssapi/python-gssapi",
     classifiers=[
-        'Development Status :: 4 - Beta',
+        'Development Status :: 5 - Production/Stable',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.3',
+        'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
         'Intended Audience :: Developers',
         'License :: OSI Approved :: ISC License (ISCL)',
         'Programming Language :: Python :: Implementation :: CPython',