py/objstr: Add support for the :_b/o/x specifier in str.format.

This groups non-decimal values by fours, such as bbb_bbbb_bbbb.  It also
supports `{:_d}` to use underscore for decimal numbers (grouped in threes).

Use of incorrect ":,b" is not diagnosed.

Thanks to @dpgeorge for the suggestion to reduce code size.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/basics/string_format.py

@@ -22,7 +22,17 @@ test("{:4o}", 123)
 test("{:4x}", 123)
 test("{:4X}", 123)
 
+test("{:4,d}", 1)
+test("{:4_d}", 1)
+test("{:4_o}", 1)
+test("{:4_b}", 1)
+test("{:4_x}", 1)
+
 test("{:4,d}", 12345678)
+test("{:4_d}", 12345678)
+test("{:4_o}", 12345678)
+test("{:4_b}", 12345678)
+test("{:4_x}", 12345678)
 
 test("{:#4b}", 10)
 test("{:#4o}", 123)