Enable mypy assignment error code

[wochinge]

[Maxime recently enabled mypy support for the first couple of issues| #6511]. In our current setup we ignore certain typing issues as there are currently too many to tackle them all at once.

We should instead tackle them step by step. [Related issue| #7513]. [Parent issue| #8243].

Current number of issues with this type: 164

Definition of Done :

• enable mypy checks for <span class="error">[assignment]</span>
• all existing <span class="error">[assignment]</span> issues are fixed

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.