Fixes#4288.
- Default imports to THIRD_PARTY, so in effect we merge the FIRST_PARTY and THIRD_PARTY stubs. This means import order is no longer affected by whether typing_extensions is installed locally.
- Treat typing_extensions, _typeshed and some others as standard library modules.
Note that isort master is very different from the latest release; we'll have to do something
different if and when the next isort release comes out.
find_module is documented to return a tuple (file, pathname,
description) where "file" is open file object and "description" a tuple
(suffix, mode, type). The type of "file" was wrong ("str" instead of
"IO[Any]") as well as that of "suffix" ("IO[Any]" instead of "str");
probably those type definitions were swapped.
Fixes#3466.
