Replace do_not_call_in_templates = True class variable in enums with instance property

[niklasmohrin]

The current pattern has the following misbehavior:

class E(enum.Enum):
    do_not_call_in_templates = True
    FOO = 42
assert set(E.__members__) == {"FOO", "do_not_call_in_templates"}

The way Django fixes this in their Choices class is by using

class E(enum.Enum):
    FOO = 42

    @property
    def do_not_call_in_templates(self):
        return True

This is technically not in line with the Django documentation, which instructs us to "set a do_not_call_in_templates attribute on the callable with the value True", but still works. This is because the Django code to check this is

getattr(E, "do_not_call_in_templates", False)

which evaluates to a <property object ...> and is therefore truthy.

Starting with Python 3.11, we should use

class E(enum.Enum):
    do_not_call_in_templates = enum.nonmember(True)`

which is also what Django's Choices uses if available.

• https://docs.djangoproject.com/en/5.1/ref/templates/api/

[Kakadus]

(as to why codecov won't cover this: django actually never executes the the property, it just checks if the attribute do_not_call_in_template (now a property object) is truthy)

[niklasmohrin]

By the way, I feel that this could be a lint for the Django ruleset of pylint of ruff or so 🤔

[richardebeling]

@niklasmohrin can you keep track of the python3.11 change to nonmember so this doesn't get lost?