Fix pypy 3.9 tox key

Author: offbyone

tox.ini

@@ -18,7 +18,7 @@ python =
     3.10 = py310, py310-numpy
     3.11 = py311, py311-numpy
     3.12 = py312, py312-numpy, lint, changelog, typing, docs
-    pypy-3 = pypy3
+    pypy3.9 = pypy3
 
 
 [tox]