Use Pypi template in debian/watch files

Starting with devscripts version 2.25.21, a template is available for 
watching releases from pypi.org sources.

This template is named Pypi and can be used as follows:

For the "foo" package listed at https://pypi.org/project/foo/, the 
method would be:

Version: 5

Template: Pypi
Dist: foo

This can be combined with other available devscripts tools such as:

Version: 5

Template: Pypi
Dist: foo
Dversion-Mangle: auto
Version-Type: STABLE_VERSION

This ensures that uscan only detects stable versions instead of beta 
versions.

For more information, see:
https://manpages.debian.org/unstable/devscripts/uscan-templates.5.en.html
https://manpages.debian.org/experimental/devscripts/debian-watch.5