use python3-venv in tox()

vars/tox.groovy

@@ -1,7 +1,7 @@
 def call() {
     tmpdir = env.TMPDIR ? env.TMPDIR : '.'
     sh """
-virtualenv -p python3 ${tmpdir}/venv/
+python3 -m venv ${tmpdir}/venv/
 ${tmpdir}/venv/bin/pip install tox
 PGPORT=`python -c 'import struct; import socket; s=socket.socket(); s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack("ii", 1, 0)); s.bind(("", 0)); print(s.getsockname()[1]); s.close()'` pg_virtualenv -o fsync=off ${tmpdir}/venv/bin/tox -rv"""
 }