Add python_requires >= 3.6

The python project still considers 3.5 supported, so we need to ensure
users on 3.5 don't get versions that they can't use.

Resolves: #198

setup.py

@@ -304,6 +304,7 @@ def gssapi_modules(lst):
     long_description=long_desc,
     license='LICENSE.txt',
     url="https://github.com/pythongssapi/python-gssapi",
+    python_requires=">=3.6.*",
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Programming Language :: Python',