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*.

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