REF: use shareable code for DTI/TDI.insert
xref #30757 should go in before this because it contains the tests.  After this, we'll be able to de-duplicate the two methods.
Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:











There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-08 18:34:47 UTC Will need an API-breaking whatsnew for changing the type of the exception.

Though to confirm: TypeErrors are typically raised when the argument has an incorrect type. In this case, the type looks correct, since it's a `Timestamp`. It's a tz-naive timestamp though, so I wouldn't expect a ValueError.

 thanks @jbrockmendel 