Single-sourcing the package version: Add instructions for setuptools using pyproject.toml (#1167)

* Add instructions for setuptools using pyproject.toml

* Copy-edit __version__ note

Co-authored-by: Brian Rutledge <brian@bhrutledge.com>

Author: justinchuby

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

@@ -36,21 +36,30 @@ 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):
+       thing by instead placing the following in the project's
+       :file:`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.
+       As of the release of setuptools 61.0.0, one can specify the
+       version dynamically in the project's :file:`pyproject.toml` file.
 
-       Also, please be aware that declarative config indicators, including the
-       ``attr:`` directive, are not supported in parameters to ``setup.py``.
+       .. code-block:: toml
+
+            [project]
+            name = "package"
+            dynamic = ["version"]
+
+            [tool.setuptools.dynamic]
+            version = {attr = "package.__version__"}
+
+       Please be aware that declarative config indicators, including the
+       ``attr:`` directive, are not supported in parameters to
+       :file:`setup.py`.
 
 #.  Use an external build tool that either manages updating both locations, or
     offers an API that both locations can use.