text: Title: mypy can't find OPT_SERIALIZE_DATACLASS
  Body: For some reason mypy doesn't think that OPT_SERIALIZE_DATACLASS exists in the orjson module.  I really don't know why this is, it's clearly defined and in the .pyi file, so maybe it's an issue with mypy?  Figured I would post it here in case you know why.
  
  My code:
  ```python
  from dataclasses import dataclass
  import orjson
  
  @dataclass
  class Test:
      value: str
  
  test = Test("hi")
  print(orjson.dumps(test, option=orjson.OPT_SERIALIZE_DATACLASS))
  ```
  
  mypy's output is " error: Module has no attribute "OPT_SERIALIZE_DATACLASS"
  