bpo-38901: Allow setting a venv's prompt to the basename of the current directory. (GH-17946)

When a prompt value of '.' is specified, os.path.basename(os.getcwd()) is used to
configure the prompt for the created venv.
diff --git a/Doc/library/venv.rst b/Doc/library/venv.rst
index 5494c0c..d778486 100644
--- a/Doc/library/venv.rst
+++ b/Doc/library/venv.rst
@@ -122,7 +122,8 @@
 
     * ``prompt`` -- a String to be used after virtual environment is activated
       (defaults to ``None`` which means directory name of the environment would
-      be used).
+      be used). If the special string ``"."`` is provided, the basename of the
+      current directory is used as the prompt.
 
     * ``upgrade_deps`` -- Update the base venv modules to the latest on PyPI