ENH: use shutil.which() instead of external which(1)
Use the `shutil.which()` function to determine whether executables exist rather than calling the external `which(1)` or `where` program. This program is not a part of POSIX base system, and modern Linux distributions are working towards making it completely optional. The builtin `shutil.which()` is available since Python 3.3 and already used in the same file.

This makes it possible to remove the dependency on `sys-apps/which` from Gentoo systems.

- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

I wasn't sure how to classify this: I chose "ENH" because I suppose removing unnecessary dependency is an "enhancement". However, this is quite trivial and I'd love to see it in 2.1.1. Ideally, your PR would make it into https://github.com/asweigart/pyperclip and then a PR here to update the pyperclip copy - but that repository seems rather inactive. Well, there's a bit of a problem with that since technically pyperclip still supports Python 2.7, and I don't think there's really point in going over the burden of implementing backwards compatibility given that the author is unresponsive for 2 years already. An option could be to first update the copy to the "latest" version. Assuming it is the last version ever to be released, clean it (remove py2 stuff), and then use shutil.which. Can then also add type annotations later.

@mroeschke Yeah agreed since pyperclip seems unmaintained, it would be best to update our vendor to the latest version before making any changes Just to be clear, by "latest" do you mean the HEAD of git repository, or the release from PyPI? I suppose I could fork the repository and make a PR with a note that it breaks py2.7 compat, if that helps. If the project is ever revived, it'll probably drop py2 support anyway.

Long term it might be a good idea to look for a maintained replacement too. I would take the latest released version from pypi as it might be more well-tested.

As the project seems inactive/dead, I personally wouldn't bother opening a PR there and instead, I would just copy the latest `__init__.py` from pypi, remove the compact with python2 , and then replace it with the current copy in pandas + run black/isort/ruff (this could be one or multiple commits) and then one/more commits with your changes on top of that.
 I've copied the changes from 1.8.2 release. Incidentally, they included the use of `shutil.which()` with Python 2 compatibility — so I've just removed the compatibility fallback, and reformatted everything. @mgorny Do you think this fixes #51744? I'm not sure whether we can test that on the CI. > @mgorny Do you think this fixes #51744? I'm not sure whether we can test that on the CI.

Looking at https://github.com/pandas-dev/pandas/issues/51744#issuecomment-1451980050, I think not. We should probably move the `WAYLAND_DISPLAY` logic outside `HAS_DISPLAY`. I can do that if you wish but I don't have a wayland setup to test it either. > Looking at [#51744 (comment)](https://github.com/pandas-dev/pandas/issues/51744#issuecomment-1451980050), I think not. We should probably move the `WAYLAND_DISPLAY` logic outside `HAS_DISPLAY`. I can do that if you wish but I don't have a wayland setup to test it either.

Probably better to keep it as-is. There is also an issue to even deprecate it #52129  Ok, thanks.

By the way, it just occurred to me to check pyperclip PR tracker, and indeed there's a fix proposed along the lines of what I thought: https://github.com/asweigart/pyperclip/pull/201. So if you prefer, I can just include that. > Ok, thanks.
> 
> By the way, it just occurred to me to check pyperclip PR tracker, and indeed there's a fix proposed along the lines of what I thought: [asweigart/pyperclip#201](https://github.com/asweigart/pyperclip/pull/201). So if you prefer, I can just include that.

Personally, I have no preference (I honestly wouldn't mind deprecating clipboard). Since this is still mostly a 1:1 copy, it might be better to make as few changes as possible. @mroeschke  Thanks @mgorny. Yeah it's best to have gradual changes in these vendored files.

FWIW, I think there is a desire to change the clipboard backend https://github.com/pandas-dev/pandas/issues/39834. At minimum, one day unvendoring pyperclip would be ideal (I'm not sure why it was vendored in the first place)