bpo-39394: Improve warning message in the re module (GH-31988)

A warning about inline flags not at the start of the regular
expression now contains the position of the flag.
(cherry picked from commit 4142961b9f5ad3bf93976a6a7162f8049e354018)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>