bpo-45413: Define "posix_venv", "nt_venv" and "venv" sysconfig installation schemes (GH-31034)

Define *posix_venv* and *nt_venv* sysconfig installation schemes
to be used for bootstrapping new virtual environments.
Add *venv* sysconfig installation scheme to get the appropriate one of the above.
The schemes are identical to the pre-existing
*posix_prefix* and *nt* install schemes.
The venv module now uses the *venv* scheme to create new virtual environments
instead of hardcoding the paths depending only on the platform. Downstream
Python distributors customizing the *posix_prefix* or *nt* install
scheme in a way that is not compatible with the install scheme used in
virtual environments are encouraged not to customize the *venv* schemes.
When Python itself runs in a virtual environment,
sysconfig.get_default_scheme and
sysconfig.get_preferred_scheme with `key="prefix"` returns
*venv*.

Doc/library/sysconfig.rst

@@ -73,7 +73,7 @@ Every new component that is installed using :mod:`distutils` or a
 Distutils-based system will follow the same scheme to copy its file in the right
 places.
 
-Python currently supports six schemes:
+Python currently supports nine schemes:
 
 - *posix_prefix*: scheme for POSIX platforms like Linux or macOS.  This is
   the default scheme used when Python or a component is installed.
@@ -83,8 +83,14 @@ Python currently supports six schemes:
 - *posix_user*: scheme for POSIX platforms used when a component is installed
   through Distutils and the *user* option is used.  This scheme defines paths
   located under the user home directory.
+- *posix_venv*: scheme for :mod:`Python virtual environments <venv>` on POSIX
+  platforms; by default it is the same as *posix_prefix* .
 - *nt*: scheme for NT platforms like Windows.
 - *nt_user*: scheme for NT platforms, when the *user* option is used.
+- *nt_venv*: scheme for :mod:`Python virtual environments <venv>` on NT
+  platforms; by default it is the same as *nt* .
+- *venv*: a scheme with values from ether *posix_venv* or *nt_venv* depending
+  on the platform Python runs on
 - *osx_framework_user*: scheme for macOS, when the *user* option is used.
 
 Each scheme is itself composed of a series of paths and each path has a unique
@@ -119,6 +125,9 @@ identifier.  Python currently uses eight paths:
       This function was previously named ``_get_default_scheme()`` and
       considered an implementation detail.
 
+   .. versionchanged:: 3.11
+      When Python runs from a virtual environment,
+      the *venv* scheme is returned.
 
 .. function:: get_preferred_scheme(key)
 
@@ -132,6 +141,10 @@ identifier.  Python currently uses eight paths:
 
    .. versionadded:: 3.10
 
+   .. versionchanged:: 3.11
+      When Python runs from a virtual environment and ``key="prefix"``,
+      the *venv* scheme is returned.
+
 
 .. function:: _get_preferred_schemes()
 

Doc/library/venv.rst

@@ -177,6 +177,11 @@ creation according to their needs, the :class:`EnvBuilder` class.
         ``clear=True``, contents of the environment directory will be cleared
         and then all necessary subdirectories will be recreated.
 
+        .. versionchanged:: 3.11
+           The *venv*
+           :ref:`sysconfig installation scheme <installation_paths>`
+           is used to construct the paths of the created directories.
+
     .. method:: create_configuration(context)
 
         Creates the ``pyvenv.cfg`` configuration file in the environment.