Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/test/test_popen2.py b/Lib/test/test_popen2.py
index cf3a094..2a15a20 100644
--- a/Lib/test/test_popen2.py
+++ b/Lib/test/test_popen2.py
@@ -49,7 +49,7 @@
     w.close()
     got = r.read()
     if got.strip() != expected:
-        raise ValueError("wrote %s read %s" % (`teststr`, `got`))
+        raise ValueError("wrote %r read %r" % (teststr, got))
     print "testing popen3..."
     try:
         w, r, e = os.popen3([cmd])
@@ -59,10 +59,10 @@
     w.close()
     got = r.read()
     if got.strip() != expected:
-        raise ValueError("wrote %s read %s" % (`teststr`, `got`))
+        raise ValueError("wrote %r read %r" % (teststr, got))
     got = e.read()
     if got:
-        raise ValueError("unexected %s on stderr" % `got`)
+        raise ValueError("unexected %r on stderr" % (got,))
     for inst in popen2._active[:]:
         inst.wait()
     if popen2._active: