bpo-37662: Documented venv.EnvBuilder.upgrade_dependencies(). (GH-15768)

(cherry picked from commit 264e034f99)

Co-authored-by: Vinay Sajip <vinay_sajip@yahoo.co.uk>

Doc/library/venv.rst

@@ -186,6 +186,14 @@ creation according to their needs, the :class:`EnvBuilder` class.
         Installs activation scripts appropriate to the platform into the virtual
         environment.
 
+    .. method:: upgrade_dependencies(context)
+
+       Upgrades the core venv dependency packages (currently ``pip`` and
+       ``setuptools``) in the environment. This is done by shelling out to the
+       ``pip`` executable in the environment.
+
+       .. versionadded:: 3.8
+
     .. method:: post_setup(context)
 
         A placeholder method which can be overridden in third party