It's quite common to have a class where we almost always have at mosta single allocated instance (per thread). Now these instances can beallocated more quickly, by reusing the memory that was used for themost recently freed instance (a separate memory block is reused foreach thread on free-threaded builds).It's used like this (only the value 1 is supported for now):```from mypy_extensions import mypyc_attr@mypyc_attr(free_list=1)class Foo:    ...```This makes a microbenchmark that only allocates and immediatelyfrees simple objects repeatedly around 3.8x faster.It's probably worth extending this to support larger free lists in thefuture.We can later look into enabling this automatically for certain nativeclasses based on profile information.

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