Add iframe support to DeepLinkResource

[cmurtaugh]

This PR modifies the DeepLinkResource class to add support for the iframe property as described in the LTI 1.3 spec. When set, this property indicates to the platform that the resource content can be embedded in a platform page using an iframe.

Usage:

    resource = DeepLinkResource()
    resource.set_url("https://my.tool/launch")\
        .set_custom_params({'my_param': my_param})\
        .set_title('My Resource')
        .set_iframe({"height": 400, "width": 800})

This PR also adjust some test settings in tox.ini and tox.yml in order to fix failing tests.

Closes #111.

[cmurtaugh]

@dmitry-viskov -- any chance you could take a look at this? Thanks!

[hmoffatt]

The spec allows a deep linking response of type link (rather than ltiResourceLink) to have an iframe field with a URL (string) field as well as width/height, so technically the t.Mapping[str, int] type isn't correct. See https://www.imsglobal.org/spec/lti-dl/v2p0#link

It would be preferable to take the unrelated pylint/workflow changes to another PR.

[dmitry-viskov]

hi @cmurtaugh .
Thanks for your effort!
I'll try to find time to review PR in the nearest future.