Adapt to removed stdlib APIs in Python 3.12. (#2170)

Closes #2137

pex/pex_builder.py

@@ -119,10 +119,12 @@ def __entry_point_from_filename__(filename):
 if '__file__' in locals() and __file__ is not None and os.path.exists(__file__):
   __entry_point__ = __entry_point_from_filename__(__file__)
 elif '__loader__' in locals():
-  from pkgutil import ImpLoader
   if hasattr(__loader__, 'archive'):
     __entry_point__ = __loader__.archive
-  elif isinstance(__loader__, ImpLoader):
+  elif hasattr(__loader__, 'get_filename'):
+    # The source of the loader interface has changed over the course of Python history from
+    # `pkgutil.ImpLoader` to `importlib.abc.Loader`, but the existence and semantics of
+    # `get_filename` has remained constant; so we just check for the method.
     __entry_point__ = __entry_point_from_filename__(__loader__.get_filename())
 
 if __entry_point__ is None:

pex/third_party/__init__.py

@@ -364,7 +364,16 @@ def uninstall(self):
                 loader.unload()
             _tracer().log("Uninstalled {}".format(self), V=3)
 
-    # The PEP-302 finder API.
+    def find_spec(self, fullname, path, target=None):
+        # Python 2.7 does not know about this API and does not use it.
+        from importlib.util import spec_from_loader  # type: ignore[import]
+
+        loader = self.find_module(fullname, path)
+        if loader:
+            return spec_from_loader(fullname, loader)
+        return None
+
+    # The Legacy PEP-302 finder API.
     # See: https://www.python.org/dev/peps/pep-0302/#specification-part-1-the-importer-protocol
     def find_module(self, fullname, path=None):
         for importable in self._importables: