Compatibility with the pykerberos package.

There are two Kerberos bindings on PyPI: kerberos and pykerberos.
PyMongo has been compatible with the kerberos package since PyMongo 2.5.0. But kerberos is not currently compatible with Python 3.
This change introduces compatibility with the new pykerberos package. Since pykerberos is compatible with Python 3, PyMongo can now use Kerberos in Python 3.

Author: ajdavis

pymongo/auth.py

@@ -87,8 +87,9 @@ def _authenticate_gssapi(credentials, sock_info, cmd_func):
         dummy, username, gsn = credentials
         # Starting here and continuing through the while loop below - establish
         # the security context. See RFC 4752, Section 3.1, first paragraph.
-        result, ctx = kerberos.authGSSClientInit(gsn + '@' + sock_info.host,
-                                                 kerberos.GSS_C_MUTUAL_FLAG)
+        result, ctx = kerberos.authGSSClientInit(
+            gsn + '@' + sock_info.host, gssflags=kerberos.GSS_C_MUTUAL_FLAG)
+
         if result != kerberos.AUTH_GSS_COMPLETE:
             raise OperationFailure('Kerberos context failed to initialize.')