work around mypy/typeshed issue with flask

a recent change to typeshed typing for Flask changed the return type of render_template from Any (implicitly, it was undefined) to Text (which is correct, render_template doesn't return a Response); this caused CI to fail, since we update mypy & typeshed each run

this can be reverted when a fix for https://github.com/python/mypy/issues/7187 is available