Typing Master Tracker
Per pandas-dev chat today opening up a master tracker for TODOs to improve typing. 

- Convert existing type hints to Python 3 syntax (#25790)
- Configure CI for type checking (assuming mypy?)
- Define base typing module for pandas (array_like, dt_like, etc...) (#25791)
- Various PRs to start adding types (by module? Maybe good sprint material)

Glancing the Python docs here's what I see for "new in version X.X.X" which we may want to consider:

**3.5.2**
 - [NewType](https://docs.python.org/3/library/typing.html#newtype)
 - [Type](https://docs.python.org/3/library/typing.html#typing.Type)
 - [DefaultDict](https://docs.python.org/3/library/typing.html#typing.DefaultDict)
 - [Text](https://docs.python.org/3/library/typing.html#typing.Text)
 - [TYPE_CHECKING](https://docs.python.org/3/library/typing.html#typing.TYPE_CHECKING)

**3.5.3**
 - [ClassVar](https://docs.python.org/3/library/typing.html#typing.ClassVar)

**3.5.4**
 - [AsyncGenerator](https://docs.python.org/3/library/typing.html#typing.AsyncGenerator)
 - [NoReturn](https://docs.python.org/3/library/typing.html#typing.NoReturn)

**3.6**
 - [Collection](https://docs.python.org/3/library/typing.html#typing.Collection)
 - [ContextManager](https://docs.python.org/3/library/typing.html#typing.ContextManager)
 - [AsyncContextManager](https://docs.python.org/3/library/typing.html#typing.AsyncContextManager)

**3.6.1**
 - [Deque](https://docs.python.org/3/library/typing.html#typing.Deque)
 - [Counter](https://docs.python.org/3/library/typing.html#typing.Counter)
 - [ChainMap](https://docs.python.org/3/library/typing.html#typing.ChainMap)

**3.7.2**
 - [OrderedDict](https://docs.python.org/3/library/typing.html#typing.OrderedDict)