add const keyword to schema object

[ismailmo1]

this adds the const keyword - see test for example

    class Body1(pydantic.BaseModel):
        name: Literal["i-am-literal"] = "i-am-literal"

results in:

{
    "title": "Body1",
    "type": "object",
    "properties": {
        "name": {
            "title": "Name",
            "type": "string",
            "default": "i-am-literal",
            "const": "i-am-literal" // new!
        }
    }
}

[ismailmo1]

mypy is currently failing (we didnt run it before) - have added issue here #7 to address it