Extracting some discussion regarding this code:

    endpoint = Unicode(
        # FIXME: Default value set temporarily to ensure we got some
        #        LTI13_ENDPOINT that can provide a response for tests.
        #
        os.getenv("LTI13_ENDPOINT", "https://canvas.instructure.com/api/lti/security/jwks"),

Is there an open-source LTI implementation that can be run with e.g. Docker to test against a real server?

[...] we could set up the IMS Global LTI 1.3 reference implementation to run tests against the "Platform" (LMS). Other options include spinning up a pre configured third party LMS that is LTI 1.3 compatible, such as Canvas or Moodle and run the integration tests with those solutions.

LTI 1.3 is an extension of OIDC / OAuth 2.0. Section 4.2 from this IMS Global article provides a good summary and a reference to the LTI 1.3 security framework which, in turn, references the RFC 7517 (JSON Web Key) standard.