TYP: typing errors for mypy==0.790
```
mypy pandas
pandas/core/computation/parsing.py:41: error: unused 'type: ignore' comment
pandas/io/common.py:452: error: Unsupported operand types for + ("List[Optional[str]]" and "List[str]")  [operator]
pandas/core/aggregation.py:483: error: Value of type variable "_LT" of "sorted" cannot be "Optional[Hashable]"  [type-var]
pandas/core/aggregation.py:606: error: Value of type variable "_LT" of "sorted" cannot be "Optional[Hashable]"  [type-var]
pandas/core/groupby/ops.py:270: error: Incompatible types in assignment (expression has type "Index", variable has type "Iterator[Any]")  [assignment]
pandas/core/groupby/ops.py:270: note: 'Index' is missing following 'Iterator' protocol member:
pandas/core/groupby/ops.py:270: note:     __next__
Found 5 errors in 4 files (checked 1057 source files)
```