on_schema_subscription needs to be a staticmethod?

[GeoBSI]

Issue

Tried implementing the on_schema_subscription as shown on the documentation: https://tartiflette.io/docs/api/directive

The app fails on _validate_directive_implementation(self) with the error message Directive Auth Method on_schema_subscription is not an Async Generator.

It works if I decorate the on_schema_subscription as a @staticmethod

Related issue: #346

Debug

Looking more in detail into the called method is_valid_async_generator(generator: AsyncGenerator), it receives my method as a
<bound method Auth.on_schema_subscription of <*******.directives.auth.auth object at 0x7f5a24a09610>>

The valid_async_generator of Tartiflette does:

isasyncgenfunction(
        generator if isfunction(generator) else generator.__call__
    )

Reproducing the logic gives:

# generator = my <bound method ****.on_schema_subscription>
isfunction(generator) = False
# so it tries with the generator.__call__
isasyncgenfunction(generator.__call__) = False

# but if I check by giving directly the generator without the isfunction check
isasyncgenfunction(generator) = True

So that explains my error

Reproduce it

Here's a simplified version of the code I'm testing issue-directive.zip

You can run it in an environment with the requirements installed with python -m src
It will fail to run if you don't remove the comment on @staticmethod in src.directives.Auth.on_schema_subscription

• Tartiflette version: 1.1.1
• Python version: 3.7
• Executed in docker: No