fix: avoid circular import of AuthzEnforcer

[MaferMazu]

Description

This PR solves #141; you can find how to reproduce the issue there.

How to test

Be able to run: ./manage lms load_policies

You can also verify that withouth this change, the import of AuthzEnforcer fails.
eduNEXT#2

Extra details

We are checking circular imports with pylint, but it does it statically. And this problem occurred at runtime; for that reason, the tests were passing.
And, casually, in test_commands.py, we imported the api first, so there was no circular import.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[mariajgrimaldi]

Can we find the root problem of these circular imports? Going back to @BryanttV's solution would be simpler: #141 (comment)

[BryanttV]

LGTM, thanks! @MaferMazu

[bmtcril]

LGTM!