Add NaTType to tslib public api
When testing the new `0.20.0` release candidate I found that the removal of `NaTType`  from the `tslib` public api breaks `odo`.

If this api breakage is actually intentional/desired I'm ok with this being closed however it should probably get a mention in the backwards incompatible API changes section of the whatsnew.

Whilst a deprecation warning is raised in https://github.com/pandas-dev/pandas/commit/648ae4f03622d8eafe1ca3b833bd6a99f56bece4#diff-d6b2c5dbe1e7fbda4ed4b87e0c49399f the removal of NaTType breaks the previous public api rendering the warning ineffective and not giving any 3rd party libraries a chance to make the necessary changes and cut a release