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.
* add compileall stubs to 3 and add types to the ones in 2
Didn't merge the stubs because all functions have additional parameters since 3.2,
so there would be no shared code between 2 and 3.
* add comment
