fix: use decorator variant with arguments

The variant of the decorator without arguments was added in Python 3.8.

Author: brandongc

reframe/core/modules.py

@@ -1031,7 +1031,7 @@ def available_modules(self, substr):
 
         return ret
 
-    @functools.lru_cache
+    @functools.lru_cache(maxsize=128)
     def conflicted_modules(self, module):
         if module.collection:
             # Conflicts have no meaning in module collection. The modules