Explicitly state that Template.__str__() is not implemented

[davepeck]

The intent of t-strings is that they are essentially always paired with a call to a function that processes them in some way.

We do not want any default behavior to convert them into strings; the only natural such behavior would be the equivalent of f-string (implemented as f() in our PEP examples) but part of the motivation for t-strings is to get away from their default behavior in cases where they're risky (sql injection, xss, etc.)