Add Python 3.10 to setup.cfg metadata

We support 3.10 as of Antelope.

Change-Id: Ic41eeb457e8e26f251471dc1de63e1f07729d468
diff --git a/setup.cfg b/setup.cfg
index 3f37df9..f224c5c 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -19,6 +19,7 @@
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
 
 [files]
 packages =