Python offers an easy-to-use framework called unittest which can be used to perform unit testing. As Joanna mentioned, the earlier testing can be implemented, the better, as that will help detect any pull requests that break some previously functioning part of the program.

Here is an introductory page on setting up unittest and constructing some basic tests:

https://docs.python.org/3/library/unittest.html

There is also spec testing, but I think that that may be beyond the scope of the project and will probably be a substantial investment of time in learning how to use it and configuring the code in order to use it.