Resolve warnings in SQLAlchemy field defs

[shreve]

Most notably, Grade.cell_type was defined as an association_proxy in the class, but was later overwritten with a column_property. Because the mapper had already been made aware of the association_proxy, this raised a warning. Grade.max_score also had a similar problem, with a column_property overwriting a regular @​property.

This commit fixes the warnings by replacing the in-class field definitions with field = None.

Additionally, remove duplicate definitions for cell_type_gradecell and cell_type_taskcell.

Fixes #1946

[github-actions]

👈 Launch a Binder on branch shreve/nbgrader/issue-1946

[brichet]

Thanks @shreve for working on this.

As far as I understand:

• cell_type and max_score attributes were always overwritten
• cell_type_from_taskcell and cell_type_from_gradecell were similar to cell_type_taskcell and cell_type_gradecell, respectively, and used only to set cell_type

If so, your change looks good to me.

[shreve]

That is my understanding as well.

• cell_type and max_score were both always overwriting the already-mapped methods/attrs in the classes, and this was detected by SQLAlchemy and firing off a soon-to-be-an-error warning.
• cell_type_from_gradecell and cell_type_from_taskcell were exactly the same code as their non-from counterparts.

[shreve]

I guess this is more maintenance than bugfix because it's cleanup and removal of a deprecation warning.

[brichet]

Thanks @shreve