Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Lib/regsub.py b/Lib/regsub.py
index 9b8fae5..0fc10a5 100644
--- a/Lib/regsub.py
+++ b/Lib/regsub.py
@@ -191,8 +191,8 @@
         fields = split(line, delpat)
         if len(fields) != 3:
             print 'Sorry, not three fields'
-            print 'split:', `fields`
+            print 'split:', repr(fields)
             continue
         [pat, repl, str] = split(line, delpat)
-        print 'sub :', `sub(pat, repl, str)`
-        print 'gsub:', `gsub(pat, repl, str)`
+        print 'sub :', repr(sub(pat, repl, str))
+        print 'gsub:', repr(gsub(pat, repl, str))