Mention enhanced attr: directive in single-sourcing option #1

Author: jwodder

source/guides/single-sourcing-package-version.rst

@@ -33,6 +33,21 @@ number of your project:
            ...
         )
 
+    .. note::
+
+       As of the release of setuptools 46.4.0, one can accomplish the same
+       thing by instead placing the following in the project's ``setup.cfg``
+       file (replacing "package" with the import name of the package):
+
+       .. code-block:: ini
+
+           [metadata]
+           version = attr:package.__version__
+
+       Earlier versions of setuptools implemented the ``attr:`` directive by
+       importing the module, but setuptools 46.4.0 added rudimentary AST
+       analysis so that ``attr:`` can function without having to import any of
+       the package's dependencies.
 
 #.  Use an external build tool that either manages updating both locations, or
     offers an API that both locations can use.