Support collection_class in relationship()

I do not know if #82 will fix the issue but I have the following problem with mypy. I use typing in order to simplify coding.

repositories: Set['Repository'] = relationship('Repository', back_populates="installation", uselist=True, collection_class=set)

This is the error I get when running mypy with sqlalchemy-stubs

error: Incompatible types in assignment (expression has type "RelationshipProperty[Iterable[Repository]]", variable has type "Set['Repository']")

I also tried with InstrumentedSet which is the instance returned by relationship in my case. but no success. The code works btw, so it’s really mypy that is not able to understand.

Do you think it’s the scope of sqlalchemy-stubs ?

Information

python 3.6
sqlalchemy 1.3.8 Database Abstraction Library
sqlalchemy-stubs 0.2 SQLAlchemy stubs and mypy plugin
├── mypy >=0.660
│   ├── mypy-extensions >=0.4.0,<0.5.0 
│   ├── typed-ast >=1.4.0,<1.5.0 
│   └── typing-extensions >=3.7.4 
│       └── typing >=3.7.4 
└── typing-extensions >=3.6.5
    └── typing >=3.7.4

[ilevkivskyi]

OK, the latter error can be fixed by adding support for collection_class. On one hand this doesn’t look like high priority (there is a simple workaround like adding correct annotation plus a # type: ignore), but on other hand this should not be hard to fix.

[ilevkivskyi]

repositories: RelationshipProperty[Set['Repository']] = relationship('Repository', back_populates="installation", uselist=True, collection_class=set)  # type: ignore