ENH: Remove Python 2 compatibility from imported pyperclip code

Remove the fallback to which/where that is only necessary for Python 2
that does not feature shutil.which().  Also collapse the imports
to avoid importing shutil.which() twice.  It is now only imported
as `_executable_exists()` to minimize the changes to the original code.

Author: mgorny

pandas/io/clipboard/__init__.py

@@ -31,8 +31,6 @@
 Cygwin is currently not supported.
 
 Security Note: This module runs programs with these names:
-    - which
-    - where
     - pbcopy
     - pbpaste
     - xclip
@@ -59,7 +57,7 @@
 )
 import os
 import platform
-from shutil import which
+from shutil import which as _executable_exists
 import subprocess
 import time
 import warnings
@@ -83,23 +81,6 @@
 
 ENCODING = "utf-8"
 
-try:
-    from shutil import which as _executable_exists
-except ImportError:
-    # 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
-        )
-
 
 class PyperclipTimeoutException(PyperclipException):
     pass
@@ -571,7 +552,7 @@ def determine_clipboard():
         return init_windows_clipboard()
 
     if platform.system() == "Linux":
-        if which("wslconfig.exe"):
+        if _executable_exists("wslconfig.exe"):
             return init_wsl_clipboard()
 
     # Setup for the macOS platform: