Trap the SSL npn protocols attribute error on Python 3.10 (#151)

only set npn if it exists

grpclib/client.py

@@ -752,10 +752,8 @@ class Channel:
         ctx.options |= (_ssl.OP_NO_TLSv1 | _ssl.OP_NO_TLSv1_1)
         ctx.set_ciphers('ECDHE+AESGCM:ECDHE+CHACHA20:DHE+AESGCM:DHE+CHACHA20')
         ctx.set_alpn_protocols(['h2'])
-        try:
+        if _ssl.HAS_NPN:
             ctx.set_npn_protocols(['h2'])
-        except NotImplementedError:
-            pass
 
         return ctx