[3.15] gh-151416: fix a borrowed ref potential use after free via fspath in os.spawnv...

[3.15] gh-151416: fix a borrowed ref potential use after free via fspath in os.spawnv/spawnve (GH-151417) (#152535)

gh-151416: fix a borrowed ref potential use after free via fspath in os.spawnv/spawnve (GH-151417)

* gh-151416: Fix use-after-free in os.spawnv/spawnve when __fspath__ mutates argv

The argv conversion loops passed references borrowed from the argv list
into fsconvert_strdup().  An item's __fspath__() can mutate the list and
release its reference to the item, leaving the converter operating on a
freed object.  A shrunk list could also make PyList_GetItem() return
NULL, which PyUnicode_FS{Converter,Decoder}() treat as a request to
release an uninitialized output variable.

Hold a strong reference to each item across the conversion, matching
parse_arglist() and parse_envlist().

* gh-151416: Don't mask non-TypeError argv conversion errors in os.spawnv

os.spawnv() replaced any error raised during argv item conversion,
such as MemoryError, codec errors, or the embedded-null ValueError,
with a generic TypeError.  Only add the contextual message when the
conversion actually raised TypeError, matching how os.spawnve() and
the exec functions propagate these errors.

The test is gated to the native C spawnv: the Python fallback used
elsewhere reports conversion failures from the forked child as exit
status 127 instead of raising.
(cherry picked from commit f57d3d6db39ea0bd39743f1a614b46cbefbfdab6)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>