BUG: Cast pd.NA to pd.NaT in to_datetime
- [x] closes #32213
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I am not fully sure we should do this, since pd.NA and pd.NaT behave differently (and we probably want to have a timestamp dtype that uses pd.NA at some point) > I am not fully sure we should do this, since pd.NA and pd.NaT behave differently (and we probably want to have a timestamp dtype that uses pd.NA at some point)

Ok, I'll close the PR, but should probably leave the issue open for now? if the user specifically calls to_datetime, it makes sense that pd.NA would cast to NaT @jbrockmendel so the reason I initially didn't add this check to `checknull_with_nat` is because this adds a dependency of tslibs on _libs/missing.pyx, while now the dependency is actually the other way around. 

We might want to make missing.pyx independent on the rest of `_libs`, so tslibs can easily depend on it. > We might want to make missing.pyx independent on the rest of _libs, so tslibs can easily depend on it.

Maybe a file with just NAType, as that doesn't depend on most of the things in libmissing (many of which do depend on tslibs).  This seems like it will be necessary if we want e.g. `pd.Timestamp(pd.NA)` to give `pd.NaT` instead of raising Is this something we need for 1.0.2, or can it wait for 1.1.0?  if the latter id like to revert and take more time to figure out the dependency structure > Is this something we need for 1.0.2, or can it wait for 1.1.0? if the latter id like to revert and take more time to figure out the dependency structure

this is a usability issue in 1.0.2

happy to change for 1.1 to a proper structure (this is already merged and backport ed)