utils: remove custom memoize decorator

[bastimeyer]

This replaces the custom memoize decorator in favor of the native functools.lru_cache decorator (available since py 3.2).

Since I didn't set maxsize=None, this also changes the cache size of each decorated method, which was infinite prior to these changes and is now set to 128 with the LRU algorithm.

Not sure why I had to force-push and explicitly set maxsize=128 on py <3.8, as it's the default there as well:
https://github.com/python/cpython/blob/3.7/Lib/functools.py#L458
https://github.com/python/cpython/blob/3.8/Lib/functools.py#L487

[bastimeyer]

Btw, could the infinite cache size of session.resolve_url be the cause of #2988 (comment) ?
This basically keeps all session instance references on the method cache, so the GC can't free it up.

[beardypig]

Btw, could the infinite cache size of session.resolve_url be the cause of #2988 (comment) ?
This basically keeps all session instance references on the method cache, so the GC can't free it up.

You might be on to something there. If it is that, then setting maxsize would solve it.