Respect ClassVar variable annotation

[viccie30]

According to PEP 526 annotated attributes in a class body are instance variables by default, unless explicitly overridden by using typing.ClassVar.

This PR makes 2 changes:

1. Honor typing.ClassVar in class variable annotations.
2. Merge the annotations in the class body and in __init__().

[pawamoy]

Hey @viccie30, thanks for the PR!

I think we need to distinguish three cases:

1. attribute is annotated with ClassVar: set class-attribute label only. If attribute is redfined in __init__, don't bother checking: that's a user error, keep the current flow (add instance-attribute label, etc.)
2. attribute is only annotated: since the attribute is not accessible through the class at runtime, but only through instances, set instance-attribute label
3. attribute is annotated and assigned: set class-attribute label, and keep the current flow (add instance-attribute label if defined again in __init__ etc.)

I think we previously handled case 3, and your PR adds support for case 1. We now just need to support case 2 🙂 WDYT? We can just check if the AnnAssign node has a value or not to decide whether to set the class-attribute or instance-attribute label.

[viccie30]

Thanks for looking into the PR.

I agree that there's 3 cases. Case 1 is user error, like you say. Case 2 is already supported in this PR, through line 566 in src/griffe/agents/visitor.py.

I'm not sure what you mean with case 3. According to the PEP if a non-ClassVar annotated attribute at class scope gets a value assigned at class scope, it's still an instance variable and not a class variable. This PR handles it as such. I have added new test cases to check this.

[pawamoy]

Thanks. Well to me z should also have the class-attribute label, because it is indeed accessible from the class at runtime. I'm more concerned about what users of the code can/will do rather than what the PEP says.

[viccie30]

I have added a branch in handle_attribute() to mark initialized annotated variables as class variables.

[pawamoy]

I've added the instance-attribute label when the attribute has a value but is not annotated as ClassVar. LGTM now, merging, thanks!