features: use ABC to denote abstract classes

capa/features/common.py

@@ -7,6 +7,7 @@
 # See the License for the specific language governing permissions and limitations under the License.
 
 import re
+import abc
 import codecs
 import logging
 import collections
@@ -96,7 +97,7 @@ class Result:
         return self.success
 
 
-class Feature:
+class Feature(abc.ABC):
     def __init__(self, value: Union[str, int, bytes], bitness=None, description=None):
         """
         Args:

capa/features/insn.py

@@ -5,6 +5,8 @@
 # Unless required by applicable law or agreed to in writing, software distributed under the License
 #  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and limitations under the License.
+import abc
+
 import capa.render.utils
 from capa.features.common import Feature
 
@@ -43,7 +45,7 @@ class Mnemonic(Feature):
 MAX_OPERAND_INDEX = 3
 
 
-class _Operand(Feature):
+class _Operand(Feature, abc.ABC):
     # superclass: don't use directly
     # subclasses should set self.name and provide the value string formatter
     def __init__(self, index: int, value: int, description=None):