Prevent memory leak with repeated calls to .schema()

[sloria]

When marshmallow Schema classes are instantiated using
type(), they get added to marshmallow's internal
class registry.

To demonstrate:

from marshmallow import class_registry
from dataclasses import dataclass
from dataclasses_json import dataclass_json

@dataclass_json
@dataclass
class Person:
    name: str

lidatong = Person('lidatong')

lidatong.schema()

assert len(class_registry._registry['marshmallow.schema.PersonSchema']) == 1

lidatong.schema()

assert len(class_registry._registry['marshmallow.schema.PersonSchema']) == 2

This is a known issue (ref marshmallow-code/marshmallow#732).

Passing a blank name prevents the class from getting
added to the registry.

[sloria]

Tests pass for me locally.

One side effect of this change is that it makes the repr and str for the schemas a bit weird:

from marshmallow import class_registry
from dataclasses import dataclass
from dataclasses_json import dataclass_json

@dataclass_json
@dataclass
class Person:
    name: str

lidatong = Person('lidatong')

print(lidatong.schema())  # <many=False>

But hopefully that's tolerable.

[sloria]

I ended up fixing the memory issue in the latest marshmallow release. So upgrading the required marshmallow (#27) should obviate this PR.

[lidatong]

Awesome! Thanks for detailing the issue and pushing through the marshmallow fix