Fix LRU cache bug with Ray

For some reason, the ray actors were failing to properly import
the cython code in the lru-dict library. Kept giving the error
`AttributeError: type object 'LRU' has no attribute '__len__'`

Switching to pylru (pure python) fixed the issues

rlmolecule/actors.py

@@ -4,7 +4,7 @@
 from typing import Any, List
 
 import ray
-from lru import LRU
+from pylru import lrucache
 
 logger = logging.getLogger(__name__)
 
@@ -32,7 +32,7 @@ class RayDictCache(DictCache):
 @ray.remote
 class RayLRUCache(DictCache):
     def __init__(self, max_size: int = int(1e5)):
-        self._dict = LRU(max_size)
+        self._dict = lrucache(max_size)
 
 
 @ray.remote

setup.cfg

@@ -15,7 +15,7 @@ include_package_data = true
 install_requires =
   graphenv
   tensorflow
-  lru-dict
+  pylru
   rdkit
   nfp