Jenkinsfile: use python3-venv

Jenkinsfile

@@ -15,7 +15,10 @@ pipeline {
                 cleanWs()
                 dir ("repository") {
                     checkout scm
-                    tox()
+                    sh """
+python3 -m venv ${env.TMPDIR}/venv/
+${env.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 ${env.TMPDIR}/venv/bin/tox -r"""
                 }
             }
             post {