tox.ini: add --namespace-packages to mypy

It is needed by latest version of types-protobuf

Also update mypy to the latest version
diff --git a/tox.ini b/tox.ini
index 0962473..8c1e704 100644
--- a/tox.ini
+++ b/tox.ini
@@ -157,9 +157,9 @@
 [testenv:mypy]
 skip_install = True
 commands =
-    mypy {posargs}
+    mypy --namespace-packages {posargs}
 deps =
-    mypy==0.910
+    mypy==0.940
     types-protobuf
     types-python-dateutil
     types-setuptools