Shell python3-config prints empty or wrong paths when its installation path contains spaces #159819

Bug report

The shell version of python-config (Misc/python-config.sh.in, installed as
bin/python3-config on POSIX platforms other than macOS) locates its own
prefix with unquoted command substitutions:

RESULT=$(dirname $(cd $(dirname "$1") && pwd -P))

With a space in the path, the substitutions are word-split. The script
complains on stderr but still exits 0 and prints an empty prefix and paths
rooted at /:

$ cp -a /tmp/py-plain "/tmp/py with spaces"
$ "/tmp/py with spaces/bin/python3-config" --prefix
/tmp/py with spaces/bin/python3-config: 24: cd: can't cd to /tmp/py
dirname: missing operand

$ echo $?
0
$ "/tmp/py with spaces/bin/python3-config" --includes
-I/include/python3.16 -I/include/python3.16

Invoked via a relative path, there is no stderr at all and the result is
silently mangled (--prefix → /tmp . spaces). The same tree at a path
without spaces works, and a script generated by
./configure --prefix="/path with spaces" fails identically in place, so the
problem is purely how the script finds itself.

Reproducer

./configure --prefix=/tmp/py-plain && make -j && make install
cp -a /tmp/py-plain "/tmp/py with spaces"
"/tmp/py with spaces/bin/python3-config" --prefix; echo "exit=$?"

Ran into this when working on relocatable builds.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS, Linux

Linked PRs

• gh-159819: Fix python-config.sh for installation paths containing spaces #159820
• [3.15] gh-159819: Fix python-config.sh for installation paths containing spaces (GH-159820) #160184
• [3.14] gh-159819: Fix python-config.sh for installation paths containing spaces (GH-159820) #160240