Python task manager from scratch, part 17: Generating some HTML

Last time I mentioned that we had two related issues to fix:

What we were displaying as HTML was not in fact valid HTML;
The code generating that "HTML" from a list of tasks was the least-tested part of Veery.

Create a folder, and a file in that folder, for functions we write to generate HTML.
Test that function.
As we test that function, realize that we're generating a list of dummy tasks over and over in our tests.
Factor out that generation into a test fixture. Put it in conftest.py.
Run Pytest to make sure that all our tests still pass.

This tutorial diverges from others (in many ways but markedly) in not using a templating engine to generate HTML for you. I'm doing this for a few reasons. First, I want to avoid dependencies. Second, I want you to understand that templating engines are string-generation (or file-generation) tools in fancy clothing. Third, I generally find templating engines frustrating to use. Fourth, when I was first learning Web development, I believe the templating stuff took up a lot of short-term memory for relatively little conceptual payoff. If we really need a templating engine later, we can always add that dependency. (As long as we behave responsibly between now and then and keep HTML-generation code cleanly separated and logically structured. Which we will.)
The test we have for the template is not sophisticated; it is just a basic "smoke test." We can always supplement it with manual checks or fancy front-end testing tools if we like. Note that this test will catch many errors that actually occur, and is annoying or impossible to write with a templating engine.