Some third-party modules are bundled with python-api inside lib.
sgtimezone contains classes for easing the conversion between the server (UTC) timezone and client timezone.
Mockgun is a ShotGrid API mocker. It's a class that has got most of the same methods and parameters that the ShotGrid API has got. Mockgun is essentially a ShotGrid emulator that (for basic operations) looks and feels like ShotGrid.
The primary purpose of Mockgun is to drive unit test rigs where it becomes too slow, cumbersome or non-practical to connect to a real ShotGrid. Using a Mockgun for unit tests means that a test can be rerun over and over again from exactly the same database state. This can be hard to do if you connect to a live ShotGrid instance.
The file shotgun_api3/lib/requirements.txt is not used to install any packages, however exists so that automated checks for CVEs in dependencies will know about bundled packages.
For this reason, it's important to add any newly bundled packages to this file, and to keep the file up to date if the bundled version of a module changes.