Fixes use of version specifiers like `python-3.12`, which:
- have an explicit `python-` prefix
- are using a major version alias that has to be resolved
  to an exact version.

Also simplified the conditional for the already
working case, since it had two branches that were virtually identical.

The directory /usr/etc is an optional directory and used by
Fedora, RHEL 9, and openSUSE. The purpose of /usr/etc is to store
distribution-provided configuration files that can be overridden
by user-modified files in /etc.

/usr/etc is not in FHS. However, as Torsten stated in his comment
(https://github.com/thkukuk/atomic-updates_and_etc/issues/2#issuecomment-524848666)
"[FHS] is currently nearly as dead as LSB, we can only ignore it
if we don't want to stop and kill all innovation."

More tailored for the emergent internal use cases

by adding an internal flag

This adds the free-threaded (without GIL) version of 3.13.0b3.

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

In exceptional cases (custom installation, malfunctions elsewhere), `pyenv-commands --sh` may return nothing.
In non-Fish, this would cause "syntax error near unexpected token `)'" in `pyenv()`.

Bash does not allow to specify a `case` option that would never match.
This works around it by defaulting to `/`. Commands, being filenames, can never match it.
In Fish, nothing needs to be done: it apparently does interpret a `case` without argument as one that never matches.

Fixes breakage in WSL with Pyenv-win installed in the host system;
doesn't affect other supported installation scenarios.

* Support ksh versions

Korn shell had two major versions: ’88 and ’93. Some systems have
ksh installed under the name `ksh93`. A few systems (maybe only
Solaris now) also have a `ksh88`. A few others use the `pdksh` (’88)
or `mksh` (’93) implementations, originated before ksh was open source.

Limit to currently-used versions

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Co-authored-by: Kevin Schoedel <kps@datatravelandexperiments.com>

* Remove all use of `greadlink`, fix #2654

Signed-off-by: Harry Chen <i@harrychen.xyz>

* revert greadlink back in tests

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Signed-off-by: Harry Chen <i@harrychen.xyz>
Co-authored-by: Anton Petrov <anton.a.petrov@gmail.com>