TYPING: Unreachable code
mypy 0.720 introduced `--warn-unreachable` http://mypy-lang.blogspot.com/2019/07/mypy-0720-released.html 

```
$ mypy pandas --warn-unreachable
pandas\core\computation\expressions.py:68: error: Statement is unreachable
pandas\core\computation\expressions.py:123: error: Statement is unreachable
pandas\core\dtypes\dtypes.py:98: error: Statement is unreachable
pandas\core\dtypes\dtypes.py:102: error: Statement is unreachable
pandas\core\dtypes\dtypes.py:316: error: Statement is unreachable
pandas\core\dtypes\dtypes.py:541: error: Right operand of 'and' is never evaluated
pandas\core\dtypes\dtypes.py:543: error: Statement is unreachable
pandas\core\indexers.py:105: error: Statement is unreachable
pandas\core\indexers.py:233: error: Statement is unreachable
pandas\core\arrays\array_.py:230: error: Statement is unreachable
pandas\core\arrays\_ranges.py:40: error: Statement is unreachable
pandas\core\arrays\period.py:243: error: Statement is unreachable
pandas\core\indexes\interval.py:1212: error: Statement is unreachable
pandas\core\computation\pytables.py:93: error: Left operand of 'and' is always true
pandas\core\computation\pytables.py:103: error: Left operand of 'and' is always true
pandas\core\indexing.py:185: error: Left operand of 'or' is always false
pandas\core\indexing.py:246: error: Statement is unreachable
pandas\core\internals\managers.py:1491: error: Statement is unreachable
pandas\core\internals\managers.py:1505: error: Statement is unreachable
pandas\core\internals\managers.py:1517: error: Statement is unreachable
pandas\io\json\_normalize.py:255: error: Statement is unreachable
pandas\io\json\_normalize.py:284: error: Statement is unreachable
(pandas-dev)
```

should we include this in `ci\code_checks.sh`?