* add typing.ContextManager for 3.6+ only
This fixes the easier part of #655.
Would it make sense to add a generic typing.ContextManager that exists in any Python version?
* update comment
* fix argument types for ContextManager.__exit__
* add AsyncContextManager
* add @asynccontextmanager
* typing.ContextManager now always exists
* back out async-related changes
Will submit those in a separate PR later
* fix import order
* AbstractContextManager only exists in 3.6+
* AbstractContextManager -> ContextManager
Also fix signature of IO.seek, IO.truncate, IO.write, and MutableMapping.update
Fixes#1016
Note: I couldn't put typing.pyi in 2and3 because of an import cycle when adding `import sys` to 2/typing.pyi
Without this change, mypy can't infer proper types for cases like
`d.get(k, [])` where it needs type context to infer the type of
`[]`. We add the value type to the second argument to `get` using
union types, and this provides the context. This doesn't affect
the effective signature of `get`, other than providing the type
context for mypy.
Also removed some related redundant method definitions where we can
just inherit the base class definition. This makes it easier to
keep the method signatures consistent.
Note that this requires a few mypy PRs before mypy will be able to
use this effectively:
* https://github.com/python/mypy/pull/2718
* https://github.com/python/mypy/pull/2715
This chage more closely matches the behavior of `get` at runtime. Users can pass whatever they want in to the default
parameter and it will be returned if the key is absent. Additionally, `get` should return an `Optional` if called with
only one parameter.
```python
z = {'a': 22}
reveal_type(z.get('b'))
reveal_type(z.get('b', 22))
reveal_type(z.get('b', 'hello'))
```
Before:
```shell
test_get_default.py:2: error: Revealed type is 'builtins.int*'
test_get_default.py:3: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Argument 2 to "get" of "dict" has incompatible type "str"; expected "int"
```
After:
```shell
test_get_default.py:2: error: Revealed type is 'Union[builtins.int*, builtins.None]'
test_get_default.py:3: error: Revealed type is 'builtins.int'
test_get_default.py:4: error: Revealed type is 'Union[builtins.int, builtins.str*]'
```
