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.
```python
import fileinput
with fileinput.input(files=('foo.txt',), inplace=True, backup='') as f:
for line in f:
print(f'prefix{line}', end='')
```
```
$ mypy test2.py
test2.py:3: error: "Iterable[str]" has no attribute "__enter__"; maybe "__iter__"?
test2.py:3: error: "Iterable[str]" has no attribute "__exit__"
```
```
$ mypy test2.py --custom-typeshed typeshed
$
```
