Closes issue bpo-5288: Allow tzinfo objects with sub-minute offsets. (#2896)

* Closes issue bpo-5288: Allow tzinfo objects with sub-minute offsets.

* bpo-5288: Implemented %z formatting of sub-minute offsets.

* bpo-5288: Removed mentions of the whole minute limitation on TZ offsets.

* bpo-5288: Removed one more mention of the whole minute limitation.

Thanks @csabella!

* Fix a formatting error in the docs

* Addressed review comments.

Thanks, @haypo.