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.

Tools/scripts/pyvenv

@@ -1,6 +1,12 @@
 #!/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