Use the random generator for item handles

[vkbo]

Summary:

This PR changes the algorithm that generates item handles to instead use the built-in random number generator. The change is trivial and the new handles are on the same format as the old, so there is no conversion needed for existing projects.

The main changes in the PR is to the test suite, which now uses a mock random generator to run most tests. The mock generator just counts upwards on each call, which maks it easier to write testes containing handles and status or importance keys.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed