BUG: Convert uint64 in maybe_convert_objects
Adds handling for `uint64` objects during conversion.  When negative numbers and `uint64` are detected, we then convert the result to `object`.

Picks up where #4845 left off. Closes #4471.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14916?src=pr) is 84.64% (diff: 100%)
> Merging [#14916](https://codecov.io/gh/pandas-dev/pandas/pull/14916?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14916   diff @@
==========================================
  Files           144        144          
  Lines         51016      51016          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43184      43185     +1   
+ Misses         7832       7831     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3ccb501...ed325cd](https://codecov.io/gh/pandas-dev/pandas/compare/3ccb50131b698bccee21780c660a70bed87396d0...ed325cdf184d4b4e28b43aeae3aeb7574a66c571?src=pr) thanks!
