Pydantic encoding issues

[leiserfg]

Describe the bug
Pydantic support is limited to types supported by json.dumps()

To Reproduce
Steps to reproduce the behavior:
Just use a BaseModel
as

class WithDate(pydantic.BaseModel):
    when : datetime

as Body in a request
it will fail because BaseModel.dict() will return {"when": datetime(...)} which the call to json.dumps() inicide of requests won't be able to handle.

Before uplink added support for pydantic I had a working implementation that was preaty similar to the one we have now, but instead of using mode.dict() I used `json.loads(model.json()), that's not the best way, but is the easier one until this get solved somehow:
pydantic/pydantic#1409

[leiserfg]

I'll make a pr to implement this without the dumps json "dance" by using a visitor, so we can use that until they provide a propper API for it on pydantic.