functools.lru_cache memory leak

[Jeroen Delcour]

tplot makes heavy use of the functools.lru_cache decorator which has a memory leak where class instances don't get garbage collected if it's used on methods (see also this video).

Since we only cache one value in all cases (lru_cache(maxsize=1)), it might as well be a cached_property instead. There's a backport available for Python <3.8.

Thanks to @dmatos2012 for telling me about this!