Improve fields type annotations a little bit.

[Reskov]

I am using mypy with an option --disallow-untyped-calls

From mypy docs

--disallow-untyped-calls
This flag reports an error whenever a function with type annotations calls a function defined without annotations.

Running command

mypy --disallow-untyped-calls .

on following code

import marshmallow as ma
class MySchema(ma.Schema):
    arb = ma.fields.Float()

gives me output

error: Call to untyped function "Float" in typed context

The same for Pluck, Method, Constant
For other ma.fields seem to be fine.