API: inference of unit64s are wrong
```
In [1]: pd.api.types.infer_dtype([2*63, -1])
Out[1]: 'integer'

In [3]: pd.api.types.infer_dtype([2*63])
Out[3]: 'integer'
```
[1] should be 'mixed' (inference which means object)
[2] should be 'uinteger'

note this will probably break some things in other parts of the suite as we sometimes infer to figure out the dtype and act on it, so 'uinteger' would then be unhandled.

e.g. in Index creation and https://github.com/pandas-dev/pandas/pull/16108/files
xref https://github.com/pandas-dev/pandas/pull/16295/files