Fix new lint warning (#11635)

Looks like there was a new release of flake8-bugbear

mypy/join.py

@@ -29,7 +29,7 @@ def join_instances(self, t: Instance, s: Instance) -> ProperType:
 
         self.seen_instances.append((t, s))
 
-        """Calculate the join of two instance types."""
+        # Calculate the join of two instance types
         if t.type == s.type:
             # Simplest case: join two types with the same base type (but
             # potentially different arguments).