Add type annotations to make_dafsa.py.

I added type narrowing in a few places (e.g. `assert node is not None`),
but only did so in places where I could add a comment explaining why
that assert was guaranteed to pass.

I did have to change semantics slightly in `top_sort` in order to get
the types to match up (and to handle empty DAFSAs as well); I added a
test that fails without those modifications.

Change-Id: I13c6d6c6c4b5c5bdb1d0597dec45d4aefd474660
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3559369
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Dylan Cutler <dylancutler@google.com>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/main@{#987016}