[3.14] gh-112527: Fix help text for required options in argparse (GH-112528) (GH-142475)

For optional arguments with required=True, the ArgumentDefaultsHelpFormatter
would always add a " (default: None)" to the end of the help text.
Since that's a bit misleading, it is removed with this commit.
(cherry picked from commit 1adb17b1a26e1547d14ca15f915e605cfdda3edd)

Co-authored-by: Fabian Henze <32638720+henzef@users.noreply.github.com>