SF bug #795506:  Wrong handling of string format code for float values.

Adding missing support for '%F'.

Will backport to 2.3.1.
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 7c98a3b..af171d0 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -550,6 +550,7 @@
 
         self.checkequal(' 42', '%3ld', '__mod__', 42)
         self.checkequal('0042.00', '%07.2f', '__mod__', 42)
+        self.checkequal('0042.00', '%07.2F', '__mod__', 42)
 
         self.checkraises(TypeError, 'abc', '__mod__')
         self.checkraises(TypeError, '%(foo)s', '__mod__', 42)