Use Python 3.8 compatible lru_cache

[Helveg]

fixes #2561

[babsey]

It still doesn't work: Another error shows up:

In [4]: from nest import spatial
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[4], line 1
----> 1 from nest import spatial

File ~/opt/nest-cache/lib/python3.8/site-packages/nest/spatial/__init__.py:27
     23 from .hl_api_spatial import *  # noqa
     24 from .hl_api_spatial import DistanceParameter as _DistanceParameter
---> 27 @_functools.lru_cache(max_size=None)
     28 def __getattr__(name):
     29     if name == "distance":
     30         return _DistanceParameter()

TypeError: lru_cache() got an unexpected keyword argument 'max_size'

[babsey]

nest.spatial works. LGTM!