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.
* Partial lib2to3 stubs
* Use typing.Text instead of str
* Add a _Path shortcut for managing PathLike
* Respond to review, pgen2/driver
* Respond to review, pgen2/grammar
* Respond to review, pgen2/parse
* Respond to review, pgen2/pgen
* Respond to review, pgen2/token
* Respond to review, pgen2/tokenize
* Respond to review, pytree
* Move to 2and3
* Make pytype happy
* Respond to review nits
* Move _RawNode, _Convert to pytree and make the latter return None
* Make concrete Symbols subclasses for python and pattern
* Add missing Callable import
