build: use mypy<1.15.0 until #799 is fixed

Author: parthea

noxfile.py

@@ -274,7 +274,9 @@ def pytype(session):
 @nox.session(python=DEFAULT_PYTHON_VERSION)
 def mypy(session):
     """Run type-checking."""
-    session.install(".[grpc,async_rest]", "mypy")
+    # TODO(https://github.com/googleapis/python-api-core/issues/799):
+    # Remove mypy constraint. We should use the latest version of mypy.
+    session.install(".[grpc,async_rest]", "mypy<1.15.0")
     session.install(
         "types-setuptools",
         "types-requests",