python update: Make deps indexable

python311 doesn't support array keys and indexing, quick fix by
converting to a list.

Fixes aspiers#7

git_explode/exploder.py

@@ -124,7 +124,7 @@ def prepare_cherrypick_base(self, sha, deps, commits):
             self.checkout_new(branch, self.base)
             return
 
-        deps = deps.keys()
+        deps = list(deps.keys())
         assert len(deps) >= 1
         self.logger.debug("  deps: %s" % ' '.join([d[:8] for d in deps]))