Fix zillion deprecation warnings in core.dtypes.common
Looking through a recent Travis log I see tons of these:

```
  /home/travis/build/pandas-dev/pandas/pandas/core/dtypes/common.py:2026: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    if dtype in [object, np.object_, 'object', 'O']:
```

This is driven by (probably not desired) cases where `dtype` is array-like at this point (also showed up in #22092).  Easily avoided by checking for hashability before checking for inclusion.