fix(ci): pin mypy for v1.3.0 release (#3988)

Author: majorgreys

riotfile.py

@@ -134,7 +134,7 @@ def select_pys(min_version=MIN_PYTHON_VERSION, max_version=MAX_PYTHON_VERSION):
             command="mypy {cmdargs}",
             create=True,
             pkgs={
-                "mypy": latest,
+                "mypy": "<0.971",
                 "types-attrs": latest,
                 "types-docutils": latest,
                 "types-protobuf": latest,