Allowing installing w/ wagtail 3.0.3

[TonisPiip]

I tested the example project w/ version 3.0.3 and it seemed to work.

tox ran and all tests passed.

Was able to create a basic form, a page, publish the page and submit the form.

4.0.* did not work due to some issue with the FormChooser when trying to add a page. Will create an issue with the stack trace.