[3.13] shell version of `python-config` started using bashisms

[mgorny]

Bug report

Bug description:

Since de2a73d python-config.sh.in is using bashisms but declaring #!/bin/sh as the shell. As a result, it now throws errors on systems using non-bash shells:

$ python3.13-config 
/usr/bin/python3.13-config: 8: [[: not found
Usage: /usr/bin/python3.13-config --prefix|--exec-prefix|--includes|--libs|--cflags|--ldflags|--extension-suffix|--help|--abiflags|--configdir|--embed

FWICS, there's no reason to use [[ ... ]] over plain shell [ ... ] there.

CC @kiendang

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-120291: Fix a bashism in python-config.sh.in #120292
• [3.13] gh-120291: Fix a bashism in python-config.sh.in (GH-120292) #120341