Explicitly export values on factory namespace

[DavidCain]

Now that factory-boy exports its types, mypy will fail in strict mode
if using the factory namespace. Specifically, --no-implicit-reexport
flags references to factory.Factory (and other similar values) as
being implicitly exported:

$ mypy --strict test.py
test.py:1:1: error: Module "factory" does not explicitly export attribute "Factory"  [attr-defined]
    from factory import Factory
    ^

We can instead explicitly export these objects. As a benefit, anybody
who uses the ill-advised from factory import * will now at least no
longer have __author__ and such pollute their namespace.

https://mypy.readthedocs.io/en/stable/config_file.html#confval-implicit_reexport