CI: mypy failures
Seeing this in multiple builds:

https://github.com/pandas-dev/pandas/runs/3972068892?check_suite_focus=true

```
mypy --version
mypy 0.910
Performing static analysis using mypy
pandas/core/dtypes/cast.py:913: error: Incompatible types in assignment (expression has type "ndarray[Any, dtype[Any]]", variable has type "NumpyArrayT")  [assignment]
pandas/tests/indexes/test_engines.py:65: error: unused "type: ignore" comment
```

Taking a look now