[3.15] gh-153300: Set global configuration variables in PyConfig_Set() (GH-153301...

[3.15] gh-153300: Set global configuration variables in PyConfig_Set() (GH-153301) (#153356)

gh-153300: Set global configuration variables in PyConfig_Set() (GH-153301)

PyConfig_Set() now also set global configuration variables. For
example, PyConfig_Set("inspect", value) now also sets Py_InspectFlag.

Use PyConfig_Set() in main.c to set the inspect flag. Python code can
now see the updated sys.flags.inspect value.
(cherry picked from commit 9fb713f1b8b774c789db007e3824c766a626ccff)

Co-authored-by: Victor Stinner <vstinner@python.org>