Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/test/test_mpz.py b/Lib/test/test_mpz.py
index dc583ae..be1fd1f 100644
--- a/Lib/test/test_mpz.py
+++ b/Lib/test/test_mpz.py
@@ -6,7 +6,7 @@
     mpz_num = mpz.mpz(num)
     vereq(int(mpz_num), num)
     vereq(long(mpz_num), num)
-    vereq(str(mpz_num), 'mpz(%s)' % `int(num)`)
+    vereq(str(mpz_num), 'mpz(%d)' % int(num))
 
 check_conversion(10)
 check_conversion(10L)