Run mypy on src/ and testing/ together

This makes testing/ actually pick up the pytest imports -- otherwise
they are opaque and we don't actually test the types.

A single run is also a bit faster and simpler. The original reason why
we split it is no longer relevant (we fixed the problems).

.pre-commit-config.yaml

@@ -47,12 +47,7 @@ repos:
     rev: v0.711
     hooks:
     -   id: mypy
-        name: mypy (src)
-        files: ^src/
-        args: []
-    -   id: mypy
-        name: mypy (testing)
-        files: ^testing/
+        files: ^(src/|testing/)
         args: []
 -   repo: local
     hooks: