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.

Author: mgorny

pandas/io/clipboard/__init__.py

@@ -79,20 +79,9 @@
 
 ENCODING = "utf-8"
 
-# The "which" unix command finds where a command is.
-if platform.system() == "Windows":
-    WHICH_CMD = "where"
-else:
-    WHICH_CMD = "which"
-
 
 def _executable_exists(name):
-    return (
-        subprocess.call(
-            [WHICH_CMD, name], stdout=subprocess.PIPE, stderr=subprocess.PIPE
-        )
-        == 0
-    )
+    return which(name) is not None
 
 
 def _stringifyText(text) -> str: