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.
The _types module can house any common type defintions used throughout
the rest of typeshed to keep defintions in sync.
First candidate is file descriptors where anything with `fileno()`
method is accepted. There were several different implementations in
various files that can be unified.
