DEPR/REGR: Fix pandas.util.testing deprecation
Closes https://github.com/pandas-dev/pandas/issues/30735

This avoids using _DeprecatedModule, which doesn't work for
direct imports from a module. Sorry for the importlib magic, but
I think this is the correct way to do things.

cc @jorisvandenbossche.
I don't have a strong opinion on this approach vs just leaving the pandas.util.testing module and importing what is needed there (and add a deprecation warning in the module, which will only trigger a warning once?). But if we keep the approach in this PR, should this be factored out in a utility and just replace the current DeprecatedModule utility? Sorry, I think @jreback is right. We don't need the import hook stuff. Just

```python
# pandas/util/testing.py
warnings.warn("pandas.util.testing is deprecated. Use pandas.testing instead")
from pandas._testing import *
```

should accomplish the same thing with less hassle. Will update shortly (I do think I need to restructure a few things to avoid a circular import from core) Sorry about that. Just using standard stuff now. LGTM thanks @TomAugspurger 