Deprecated distutils module in Python 3.12

For 3.0.x, alias shutil.which() as find_executable(); load distutils.find_executable() as itself on Python 2.
Reverts and replaces fcf0ba9, which had erroneous indentation.

bin/mrtrix3.py

@@ -15,7 +15,6 @@
 # For more details, see http://www.mrtrix.org/.
 
 import os, sys
-from distutils.spawn import find_executable
 
 try:
   # since importlib code below only works on Python 3.5+

lib/mrtrix3/path.py

@@ -21,13 +21,18 @@
 
 
 import ctypes, errno, inspect, os, random, string, subprocess, time
-from distutils.spawn import find_executable
 # Function can be used in isolation if potentially needing to place quotation marks around a
 #   filesystem path that is to be included as part of a command string
 try:
   from shlex import quote
 except ImportError:
   from pipes import quote
+# Distutils removed in 3.12, but shutil.which not available in 2.7
+try:
+  from shutil import which as find_executable
+except ImportError:
+  from distutils.spawn import find_executable
+
 from mrtrix3 import CONFIG
 from mrtrix3.utils import STRING_TYPES
 

lib/mrtrix3/run.py

@@ -17,10 +17,15 @@
 # pylint: disable=unspecified-encoding
 
 import collections, itertools, os, shlex, signal, string, subprocess, sys, tempfile, threading
-from distutils.spawn import find_executable
 from mrtrix3 import ANSI, BIN_PATH, COMMAND_HISTORY_STRING, EXE_LIST, MRtrixBaseError, MRtrixError
 from mrtrix3.utils import STRING_TYPES
 
+# Distutils removed in 3.12, but shutil.which not available in 2.7
+try:
+  from shutil import which as find_executable
+except ImportError:
+  from distutils.spawn import find_executable
+
 IOStream = collections.namedtuple('IOStream', 'handle filename')