mypy

9637f992 - Fix crash with assignment to variable guarded with TypeGuard (#10683)

Fix crash with assignment to variable guarded with TypeGuard (#10683) This is a quick fix to unblock the 0.910 release. The type guard type can be unrelated to the original type, so we shouldn't join it. I'm not sure whether this is right from first principles, but it seems to address the issue. Fixes #10671.