autodoc now attempts to import with `typing.TYPE_CHECKING = True`
first, and then falls back to `typing.TYPE_CHECKING = False` if that
fails. Unfortunately, the first import can leave behind some
partially-imported modules in `sys.modules` such that the retry fails.

Attempt to work around this by detecting what modules were added to
`sys.modules` by the first attempt and removing them before retrying.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>