Use workaround to avoid staticmethod 3.10/Cython issue (rapidsai#5202)

With Python 3.10, there appears to be an issue with the interaction between the staticmethod decorator and Cython. This workaround temporarily switches all staticmethods in FIL to classmethods until the underlying issue can be sorted.

Resolve rapidsai#5200.

Authors:
  - William Hicks (https://github.com/wphicks)

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: rapidsai#5202

python/cuml/fil/fil.pyx

@@ -116,13 +116,13 @@ cdef class TreeliteModel():
         TreeliteQueryNumFeature(self.handle, &out)
         return out
 
-    @staticmethod
-    def free_treelite_model(model_handle):
+    @classmethod
+    def free_treelite_model(cls, model_handle):
         cdef uintptr_t model_ptr = <uintptr_t>model_handle
         TreeliteFreeModel(<ModelHandle> model_ptr)
 
-    @staticmethod
-    def from_filename(filename, model_type="xgboost"):
+    @classmethod
+    def from_filename(cls, filename, model_type="xgboost"):
         """
         Returns a TreeliteModel object loaded from `filename`
 
@@ -173,8 +173,9 @@ cdef class TreeliteModel():
         filename_bytes = filename.encode("UTF-8")
         TreeliteSerializeModel(filename_bytes, self.handle)
 
-    @staticmethod
-    def from_treelite_model_handle(treelite_handle,
+    @classmethod
+    def from_treelite_model_handle(cls,
+                                   treelite_handle,
                                    take_handle_ownership=False):
         cdef ModelHandle handle = <ModelHandle> <size_t> treelite_handle
         model = TreeliteModel(owns_handle=take_handle_ownership)
@@ -843,8 +844,9 @@ class ForestInference(Base,
         )
         return cuml_fm
 
-    @staticmethod
-    def load(filename,
+    @classmethod
+    def load(cls,
+             filename,
              output_class=False,
              threshold=0.50,
              algo='auto',