Altgraph: revert our patches and use upstream.

Essentially this PR reverts upstream patches so that we can delete the altgraph files and use upstream.

Author: htgoebel

PyInstaller/depend/analysis.py

@@ -468,8 +468,8 @@ def importer_names(self, name):
         node = self.findNode(name)
         if node is None : return []
         _, iter_inc = self.get_edges(node)
-        return [importer.identifier for importer in iter_inc]
-
+        return [importer.identifier for importer in iter_inc
+                if importer is not None]
 
     # TODO create class from this function.
     def analyze_runtime_hooks(self, custom_runhooks):