Rewrite test_build_page_empty test

[mondeja]

• Mock the jinja2.environment.Template.render method to prevent this error.
• I'm using here assert_called_once instead of self.assert_mock_called_once because is Python >= 3.6 compatible (see Drop assert_mock_called_once Python < 3.6 compat method from tests #2611).

[pawamoy]

https://stackoverflow.com/a/49718531/3451029

You can't mock the second argument of isinstance(), no. The documentation you found concerns making a mock as the first argument pass the test. If you want to produce something that is acceptable as the second argument to isinstance(), you actually have to have a type, not an instance (and mocks are always instances).
You could use a subclass instead of MyClass instead

So instead of mock.patching jinja.environment.Template, we could try to subclass it:

...
class Template(jinja.environment.Template):
    def render(*args, **kwargs):
        return ""

jinja.environment.Template = Template
...

[mondeja]

I think the easiest here is to just mock up the render method.