cache.py: Avoid dict changed size during iteration

Subset of internetarchive#4330

This code is ___only used in testing___ but it raises a RuntimeError on Apple Silicon M1 Macs.

See internetarchive#4330 Screenshots, problem 4 for details.

openlibrary/core/cache.py

@@ -157,7 +157,7 @@ def join_threads(self):
 
         Used only in testing.
         """
-        for name, thread in self.active_threads.items():
+        for name, thread in list(self.active_threads.items()):
             thread.join()
 
     def encode_args(self, args, kw=None):