Add support for importing data files

Adds support for importing data files. Data files should be placed in data/sample or data/secret for each assignment. Data in the sample folder will be accessible to students, the secret directory has its contents marked as specTestOnly.

To support the new := operator, the required Python version was increased to 3.8.

To use the files, use from generator_utils import open. This definition of open overrides the built in one to look for files in the data/sample and data/secret directories. Only providing the filename suffices. The open object redirects any calls to the opened file, so all other behaviour remains the same.