Factor out venv-installing code into a module.

Factor out the code in tools/provision.py which installs a python2
and python3 venv into a module (tools/setup/setup_venvs.py) which
can also be used as a script.

tools/setup/setup_venvs.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python
+
+import os
+import sys
+from os.path import dirname, abspath
+
+ZULIP_PATH = dirname(dirname(dirname(abspath(__file__))))
+if ZULIP_PATH not in sys.path:
+    sys.path.append(ZULIP_PATH)
+
+from scripts.lib.setup_venv import setup_virtualenv
+
+def main():
+    # type: () -> None
+    PY2_DEV_REQS_FILE = os.path.join(ZULIP_PATH, "requirements", "py2_dev.txt")
+    setup_virtualenv("/srv/zulip-venv", PY2_DEV_REQS_FILE)
+
+    PY3_DEV_REQS_FILE = os.path.join(ZULIP_PATH, "requirements", "py3_dev.txt")
+    setup_virtualenv("/srv/zulip-py3-venv", PY3_DEV_REQS_FILE, virtualenv_args=['-p', 'python3'])
+
+if __name__ == "__main__":
+    main()