Add generic functionality (#595), fix #556

* Added generic functionality

* Skip tests in python 3.6

* double quote -> single quote

* Simplified, with more comprehensive tests

* double quote -> single quote

* Remove unintentional file

* Add caching

* don't cythonize generics.py

* Make work with mypy

* Remove __parameters__

* double quote -> single quote i'll remember one day

* More cleanup and validation

* Removed unwanted file

* A little more cleanup, and finish the PR

* Add proper inheritance

* Added note about inheritance to docs

* Added error for double-parameterizing

* Should build for python3.7

* Works with both 3.6 and 3.7

* Fixed bug with caching for single argument

* handle __name__ for generic models

* double quote -> single quote

* Updated error messages

docs/examples/generics.py

@@ -0,0 +1,61 @@
+from typing import Generic, TypeVar, Optional, List
+
+from pydantic import BaseModel, validator, ValidationError
+from pydantic.generics import GenericModel
+
+
+DataT = TypeVar('DataT')
+
+
+class Error(BaseModel):
+    code: int
+    message: str
+
+
+class DataModel(BaseModel):
+    numbers: List[int]
+    people: List[str]
+
+
+class Response(GenericModel, Generic[DataT]):
+    data: Optional[DataT]
+    error: Optional[Error]
+
+    @validator('error', always=True)
+    def check_consistency(cls, v, values):
+        if v is not None and values['data'] is not None:
+            raise ValueError('must not provide both data and error')
+        if v is None and values.get('data') is None:
+            raise ValueError('must provide data or error')
+        return v
+
+
+data = DataModel(numbers=[1, 2, 3], people=[])
+error = Error(code=404, message='Not found')
+
+print(Response[int](data=1))
+# > Response[int] data=1 error=None
+print(Response[str](data='value'))
+# > Response[str] data='value' error=None
+print(Response[str](data='value').dict())
+# > {'data': 'value', 'error': None}
+print(Response[DataModel](data=data).dict())
+# > {'data': {'numbers': [1, 2, 3], 'people': []}, 'error': None}
+print(Response[DataModel](error=error).dict())
+# > {'data': None, 'error': {'code': 404, 'message': 'Not found'}}
+
+try:
+    Response[int](data='value')
+except ValidationError as e:
+    print(e)
+"""
+4 validation errors
+data
+  value is not a valid integer (type=type_error.integer)
+data
+  value is not none (type=type_error.none.allowed)
+error
+  value is not a valid dict (type=type_error.dict)
+error
+  must provide data or error (type=value_error)
+"""