Use-after-free in os.spawnv/spawnve when `__fspath__` mutates argv

[gpshead]

Bug report

Bug description:

os.spawnv() and os.spawnve() on Windows convert each argv item with
fsconvert_strdup(), passing a reference borrowed from the argv list via
PyList_GetItem(). The conversion runs the item's .__fspath__() method. If
that method drops the list's last reference to the item, the converter
operates on a freed object -- a use-after-free.

A second crash mode: if .__fspath__() shrinks the list, PyList_GetItem()
returns NULL for a later index, and fsconvert_strdup(NULL, out) makes
PyUnicode_FS{Converter,Decoder}() release the uninitialized output
variable.

Same family as gh-151403 (_posixsubprocess.fork_exec) and gh-143309
(parse_envlist). Triggering requires a custom __fspath__, so this is
crash hardening with no security impact.

The fix is to hold a strong reference to each item across the conversion,
matching parse_arglist() and parse_envlist().

(PR ready, i'll attach that shortly)

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

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