web: Improve typing for UIModule.render

web: Improve typing for UIModule.render

In practice, UIModule.render often returns the
result of self.render_string, which returns bytes.
In fact, we have an example of that in this file
which had a type ignore comment. UIModule.render
may now return either str or bytes and downstream
code is responsible for handling this. (Note that
the new call to _unicode appears to be redundant
since the Template module's bytes return was
already working correctly, but this conversion is
necessary to satisfy the type checker.)

Fixes #3050