[3.12] gh-109475: Fix support of explicit option value "--" in argparse (GH-114814) (GH-115036)

For example "--option=--".
(cherry picked from commit 4aa4f0906df9fc9c6c6f6657f2c521468c6b1688)

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