adjust isort config (#4290)

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.

.travis.yml

@@ -32,7 +32,6 @@ jobs:
     - name: "black"
       install: pip install $(grep black requirements-tests-py3.txt)
       script: black --check --diff stdlib third_party
-  allow_failures:
     - name: "isort"
       install: pip install $(grep isort requirements-tests-py3.txt)
       script: isort --check-only --diff --recursive stdlib third_party