tox: Run Python 3.10 tests in Python 3.10 environment

Correct a typo introduced when we added 3.10 support.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: dda1112 ("Add Python 3.10 support, drop Python 3.6 support")

Author: stephenfin

tox.ini

@@ -83,4 +83,4 @@ python =
     3.7: py37
     3.8: py38
     3.9: py39
-    3.10: py39
+    3.10: py310