Actually import set_encoding into the main module

Previously, `set_encoding` was only available from
`gssapi._utils`.  However, `gssapi._utils` was not
intended for external use, while `set_encoding` is
intended for external use.

Now, `set_encoding` is available from the main
`gssapi` module.

Author: DirectXMan12

gssapi/__init__.py

@@ -6,4 +6,6 @@
 from gssapi.names import Name  # noqa
 from gssapi.sec_contexts import SecurityContext  # noqa
 
+from gssapi._utils import set_encoding  # noqa
+
 """The High-Level GSSAPI Wrapper"""