Use built-in shutil.which() instead of which(1) tool

Use the built-in shutil.which() function that is available since Python
3.3 instead of the external which(1) program.  The latter is not part
of POSIX and may be missing entirely (Gentoo is activately working
towards removing it)>

Signed-off-by: Michał Górny <mgorny@gentoo.org>

k5test/realm.py

@@ -68,16 +68,13 @@ def _cfg_merge(cfg1, cfg2):
 
 
 def _discover_path(name, default, paths):
-    stderr_out = subprocess.DEVNULL
-    try:
-        path = subprocess.check_output(["which", name], stderr=stderr_out).strip()
-        path = path.decode(sys.getfilesystemencoding() or sys.getdefaultencoding())
+    path = shutil.which(name)
+    if path is not None:
         _LOG.debug(f"Using discovered path for {name} ({path})")
-        return path
-    except subprocess.CalledProcessError as e:
+    else:
         path = paths.get(name, default)
         _LOG.debug(f"Using default path for {name} ({path}): {e}")
-        return path
+    return path
 
 
 class K5Realm(metaclass=abc.ABCMeta):