chore: unpin mypy check version

Author: daniel-sanche

gapic/templates/noxfile.py.j2

@@ -91,8 +91,7 @@ nox.options.error_on_missing_interpreters = True
 def mypy(session):
     """Run the type checker."""
     session.install(
-        # TODO(https://github.com/googleapis/gapic-generator-python/issues/2410): Use the latest version of mypy
-        "mypy<1.16.0",
+        "mypy",
         "types-requests",
         "types-protobuf",
     )