py, build: drop py plugin from core & require external package

End of support for Python 2.7 on Google App Engine, in January 2024,
approaches. Dropping this plugin from core now will let us make the
transition to a new backend easier, without having to fit it into
Sopel's release cadence.

Marked the requirement with a TODO for 8.1, when it should become an
optional manually-installed plugin like the others that have been
packageized (ipython, spellcheck, etc.).

Author: dgw

pyproject.toml

@@ -54,6 +54,7 @@ dependencies = [
     "importlib_metadata>=3.6",
     "packaging",
     "sopel-help>=0.4.0",
+    "sopel-py>=1.0.0",  # TODO: drop for 8.1
 ]
 
 [project.urls]