Ending Python module files with .py is sufficiently established in the broader Python community that it's just noise to repeat it in our own developer guide (unlike C++, there are not multiple competing extensions).
Ending Python script files with .py is not as well-established, but I believe it's actually a minor anti-pattern, as it exposes an implementation detail (the language the script is written in) to a caller who shouldn't care. And while it's unlikely that most of our scripts will ever be in a language other than Python, we should not make that more difficult for no reason.
At the same time, we already have many scripts that already end in .py, and establishing a rule that says they shouldn't have a suffix could result in exactly the sort of churn we want to avoid: DM has a well-established history of not being able to avoid churning existing code to meet new standards when the standards change.
I therefore propose that we drop mention of suffixes for Python files from the developer guide entirely. The new butler and pipetask commands do not have a .py suffix (will will continue to if this RFC is accepted), and are already slated to replace all of the .py-ending CmdLineTasks; I think that will make the future consistent enough.