ENH: add support for datetime.date/time in to_sql (GH6932)

Closes #6932

Support for writing `datetime.date` and `datetime.time` object columns with `to_sql`.

Works nicely for the sqlalchemy mode, for sqlite fallback it writes OK, but comes back as object strings (this is also the case for datetime)

Remaining question:
* when reading the data back in `TIME` columns are converted back to `datetime.time`, but `DATE` columns are converted to `datetime64` and not `datetime.date`. Is this OK? Or better try to convert to `datetime.date`?

Still have to add tests.

yep you can certainly test for the datetime64 first . not sure exactly what you need. take a look at ``core/index.py/Index.__new__``, which goes thru a bunch of steps to figure out what is what. You  have an advantage, you already know its a Series, and already have it coerced (if its possible), so for example if it is ``datetime64[ns]`` you are done. (as you are already doing).