We had one place where we were defining a TypeAlias using the

newer-style list[...] (and related), which doesn't work for Python

3.8. This works fine for type annotations on all versions, but for type

aliases, it's a runtime error. Luckily this was hidden behind a check

for TYPE_CHECKING so it didn't break production.
This change also moves a couple more things into the TYPE_CHECKING

block.

Ran unit tests.