utils: imp module is removed in Python 3.12 - get is_frozen() from _imp

imp has been deprecated for a long time, and has finally been removed in Python
3.12 .

The successor importlib is using the same internal _imp module as imp, but
doesn't expose it's is_frozen. Using the internal function directly seems like
the cleanest solution.

Another alternative to
  imp.is_frozen("__main__")
is
  sys.modules['__main__'].__spec__.origin == 'frozen'
but that seems even more internal and fragile.

mercurial/utils/resourceutil.py

@@ -8,7 +8,7 b''
 # GNU General Public License version 2 or any later version.
 
 
-import imp
+import _imp
 import os
 import sys
 
@@ -24,7 +24,7 b' def mainfrozen():'
     return (
         pycompat.safehasattr(sys, "frozen")  # new py2exe
         or pycompat.safehasattr(sys, "importers")  # old py2exe
-        or imp.is_frozen("__main__")  # tools/freeze
+        or _imp.is_frozen("__main__")  # tools/freeze
     )