ERR: raise on python in object hashing, only supporting strings, nulls
xref #14729 
cc @mikegraham 
cc @mrocklin
 Awesome, I think this is a really worth while restriction. Thanks! Initially you were worried about things like tuples for multiindices -- did you decide that wasn't really a concern? problem with tuples is that they can contain arbitrary things - so punting for now ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14767?src=pr) is 85.27% (diff: 100%)
> Merging [#14767](https://codecov.io/gh/pandas-dev/pandas/pull/14767?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
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  Hits          43418      43418          
  Misses         7497       7497          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [423c16a...8fb4ed0](https://codecov.io/gh/pandas-dev/pandas/compare/423c16a2ee88d82202c1e6b24a31d47ec6a04b82...8fb4ed0adf8f892bf7499ed7d218107864d274ea?src=pr)