[3.11] gh-94207: Fix struct module leak (GH-94239) #94265
                
     Merged
            
            
          
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Make _struct.Struct a GC type
This fixes a memory leak in the _struct module, where as soon
as a Struct object is stored in the cache, there's a cycle from
the _struct module to the cache to Struct objects to the Struct
type back to the module. If _struct.Struct is not gc-tracked, that
cycle is never collected.
This PR makes _struct.Struct GC-tracked, and adds a regression test.
(cherry picked from commit 6b86534)
Co-authored-by: Mark Dickinson mdickinson@enthought.com