Remove mypy version upper bound in setup.py

Due to a bug in mypy 0.940 (#870), we made two changes in #871:

• pinned mypy==0.931 in requirements.txt (for running our tests);
• bounded mypy<0.940 in setup.py (for downstream users).

After the mypy bug was quickly fixed upstream in 0.941, our setup.py
bound has been repeatedly raised but not removed (#886, #939, #973).
The only changes in those commits have been to the precise wording of
error messages expected in our tests.  Those wording changes don’t
impact compatibility for downstream users, so it should be safe to go
back to allowing them to upgrade mypy independently.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

setup.py

@@ -20,7 +20,7 @@ def find_stub_files(name: str) -> List[str]:
     readme = f.read()
 
 dependencies = [
-    "mypy>=0.930,<0.970",
+    "mypy>=0.930",
     "django",
     "django-stubs-ext>=0.4.0",
     "tomli",