bug#31410: 25.3; Python.el: Support for Python virtual environment (venv)

[seb@k-7.ch (Sébastien Gendre)]

When reading Python.el source code, I see it's support Python virtual
environment by setting the variable 'python-shell-vertualenv-root'

It would be more discoverable if we have an interactive function to do
that. Somthing like 'python-virtualenv' or similar. The must would be to
have a key binding for it and eplained it in mode help.



Emacs informations:
In GNU Emacs 25.3.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.22.26)
 of 2018-01-12 built on buildhw-08.phx2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.11906000
Configured using:
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Configured features:
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ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
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Important settings:
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Major mode: Org

Minor modes in effect:
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