py/objstr: Support '{:08}'.format("Jan") like Python 3.10.

The new test has an .exp file, because it is not compatible with Python 3.9
and lower.

See CPython version of the issue at https://bugs.python.org/issue27772

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

tests/basics/string_format_error.py

@@ -1,7 +1,7 @@
 # tests for errors in {} format string
 
 try:
-    '{0:0}'.format('zzz')
+    '{0:=}'.format('zzz')
 except (ValueError):
     print('ValueError')