add a py3 venv in dev environment

This allows us to run e.g. tests in a different python version:
$ venv3/bin/py.test test/netlib/test_tcp.py

.gitignore

@@ -2,6 +2,7 @@
 MANIFEST
 */tmp
 /venv
+/venv3
 *.py[cdo]
 *.swp
 *.swo

dev.sh

@@ -1,13 +1,29 @@
-#!/bin/bash
+#!/bin/sh
 set -e
-VENV=./venv
+
+VENV="./venv"
+VENV3="${VENV}3"
 
 python -m virtualenv $VENV --always-copy
 . $VENV/bin/activate
-pip install -U pip setuptools
-pip install -r requirements.txt
+pip install -q -U pip setuptools
+pip install -q -r requirements.txt
 
 echo ""
-echo "* Created virtualenv environment in $VENV."
-echo "* Installed all dependencies into the virtualenv."
-echo "* You can now activate the virtualenv: \`. $VENV/bin/activate\`"
+echo "* Virtualenv created in $VENV and all dependencies installed."
+echo "* You can now activate the $(python --version) virtualenv with this command: \`. $VENV/bin/activate\`"
+
+if $(python --version 2>&1 | grep -q "Python 2.") && command -v python3 >/dev/null 2>&1; then
+	echo ""
+	echo ""
+
+  python3 -m virtualenv "$VENV3" --always-copy
+	. "$VENV3/bin/activate"
+	pip install -q -U pip setuptools
+	pip install -q -r requirements.txt
+
+	echo ""
+	echo "* Virtualenv created in $VENV3 and all dependencies installed."
+  echo "* You can now activate the $(python --version) virtualenv with this command: \`. $VENV3/bin/activate\`"
+fi
+