Issue #25154: Deprecate the pyvenv script.

This was done so as to move users to `python3 -m venv` which prevents
confusion over which Python interpreter will be used in the virtual
environment when more than one is installed.

Doc/whatsnew/3.6.rst

@@ -197,7 +197,10 @@ Deprecated functions and types of the C API
 Deprecated features
 -------------------
 
-* None yet.
+* The ``pyvenv`` script has been deprecated in favour of ``python3 -m venv``.
+  This prevents confusion as to what Python interpreter ``pyvenv`` is
+  connected to and thus what Python interpreter will be used by the virtual
+  environment. See :issue:`25154`.
 
 
 Removed

Misc/NEWS

@@ -289,6 +289,12 @@ Windows
 
 - Issue #25022: Removed very outdated PC/example_nt/ directory.
 
+Tools/Demos
+-----------
+
+- Issue #25154: The pyvenv script has been deprecated in favour of
+  `python3 -m venv`.
+
 
 What's New in Python 3.5.1 release candidate 1?
 ===============================================

Tools/scripts/pyvenv

 #!/usr/bin/env python3
 if __name__ == '__main__':
     import sys
+    import pathlib
+
+    executable = pathlib.Path(sys.executable or 'python3').name
+    print('WARNING: the pyenv script is deprecated in favour of '
+          '`{} -m venv`'.format(exeutable, file=sys.stderr))
+
     rc = 1
     try:
         import venv