Support Sanic 21 and later

[jwhitlock]

Sanic 21.12 will require changes in tests, and possibly in code.

Sanic 21.3 moved testing support from sanic.testing to a new module sanic-testing. This will require a different requirements file than Sanic 20.x to install sanic-testing.

There are other changes needed to support sanic-testing, such as changes to app registration and the test client. The changes are large enough that a new test file may be useful, rather than dynamically changing code based on the version.

There may be other changes in the framework itself. For example, this test:

python-dockerflow/tests/sanic/test_sanic.py

Lines 98 to 99 in 5e30d69

	dockerflow = Dockerflow()
	assert "dockerflow.heartbeat" not in app.router.routes_names

fails with AttributeError: 'Router' object has no attribute 'routes_names'.

Currently, tests pass for Sanic 20.12.3, which is supported until December 2022. After this date, if we don't support Sanic 21.12, then we may want to drop Sanic support entirely.

[jwhitlock]

@relud - do you have time and interest to work on this?

[relud]

i'll try to make time to fix this up when i return from vacation next week