Specify language level to 3.

cython 0.29.21 complains:

FutureWarning: Cython directive 'language_level' not set, using 2 for now
(Py2). This will change in a later release!

Author: enkelli

sasl/saslwrapper.pyx

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+#cython: language_level=3
+
 from six import string_types, PY3
 
 from libcpp cimport bool