bpo-45162: Revert "Remove many old deprecated unittest features" (GH-30935)

Revert "bpo-45162: Remove many old deprecated unittest features (GH-28268)"

This reverts commit b0a6ede3d0bd6fa4ffe413ab4dfc1059201df25b.

We're deferring this change until 3.12 while upstream projects that use
the legacy assertion method names are fixed.  See the issue for links
to the discussion. Many upstream projects now have issues and PRs
filed.