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 stubs for Python 2 profile and cProfile
Partially fixes#531. Contrary to the Python 2.7 docs at
https://docs.python.org/2/library/profile.html#module-cProfile,
these modules do not have exactly the same interface. For
example, profile.Profile() does not have an enable() method.
* profile stubs: move to 2and3, add __init__, use private names for typevars
