re6st: fixes in rebootstrap script

- Make it compatible with Python 3.
- Install setuptools in a way that's compatible with slapos.rebootstrap
  (otherwise, rebootstrapping fails because signature dependency to
  setuptools changes).

obs/re6st/rebootstrap

-#!/usr/bin/python2 -S
-import glob, os, sys, tarfile, zipfile
-d = "download-cache/dist"
-p = "setuptools-*"
-x, = glob.glob(d + "/" + p)
-if x.endswith(".zip"):
-    zipfile.ZipFile(x).extractall()
-else:
-    tarfile.TarFile.open(x, "r:*").extractall()
-sys.path[:0] = p = glob.glob(p)
-from setuptools.command.easy_install import main
-for x in "bin", "eggs":
-    try:
-        os.mkdir(x)
-    except OSError, e:
-        pass
-main(["-f", d, "-mxd", x, "zc.buildout"])
-p.append(*glob.glob("eggs/zc.buildout-*"))
-os.write(os.open("bin/buildout", os.O_CREAT|os.O_WRONLY|os.O_TRUNC, 0777), """\
+#!/usr/bin/python -S
+import glob, os, shutil, sys, tarfile, tempfile, zipfile
+dist = "download-cache/dist"
+tmp = tempfile.mkdtemp()
+try:
+    setuptools, = glob.glob(dist + "/setuptools-*")
+    if setuptools.endswith(".zip"):
+        zipfile.ZipFile(setuptools).extractall(tmp)
+    else:
+        tarfile.TarFile.open(setuptools, "r:*").extractall(tmp)
+    x, = os.listdir(tmp)
+    sys.path.insert(0, os.path.join(tmp, x))
+    from setuptools.command.easy_install import main
+    for x in "bin", "eggs":
+        try:
+            os.mkdir(x)
+        except OSError:
+            pass
+    main(["-f", dist, "-mxd", x, setuptools, "zc.buildout"])
+finally:
+  shutil.rmtree(tmp)
+
+with os.fdopen(os.open("bin/buildout", os.O_CREAT|os.O_WRONLY|os.O_TRUNC, 0o777), 'w') as f:
+    f.write("""\
 #!%s -S
 import os, sys
 d = os.path.dirname(os.path.abspath(os.path.realpath(__file__)))
-d = os.path.dirname(d) + os.sep
+d = os.path.dirname(d) + %r
 sys.path[:0] = (%s)
 from zc.buildout.buildout import main
 sys.exit(main())
-""" % (sys.executable, ", ".join("d + %r" % p for p in p)))
+""" % (
+    sys.executable,
+    "/%s/" % x,
+    ", ".join(map("d + %r".__mod__, os.listdir(x))),
+    ))