Drop support for Python 3.9 (#12633)

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 4.2.0
-envlist = py{39,310,311,312,313}
+envlist = py{310,311,312,313}
 
 [testenv]
 usedevelop = True
@@ -19,7 +19,7 @@ passenv =
     BUILDER
     READTHEDOCS
 description =
-    py{39,310,311,312,313}: Run unit tests against {envname}.
+    py{310,311,312,313}: Run unit tests against {envname}.
 extras =
     test
 setenv =