fix: base_extractor.py: __metaclass__ is Python 2 syntax, ignored in Py3

Closes #3024

CHANGELOG.md

@@ -34,6 +34,8 @@
 
 - fix: elffile.py: get_base_address returns None instead of NO_ADDRESS @williballenthin #3023
 
+- fix: base_extractor.py: __metaclass__ is Python 2 syntax, ignored in Py3 @williballenthin #3024
+
 ### capa Explorer Web
 
 ### capa Explorer IDA Pro plugin

capa/features/extractors/base_extractor.py

@@ -91,7 +91,7 @@ class InsnHandle:
     inner: Any
 
 
-class StaticFeatureExtractor:
+class StaticFeatureExtractor(abc.ABC):
     """
     StaticFeatureExtractor defines the interface for fetching features from a
     sample without running it; extractors that rely on the execution trace of
@@ -109,8 +109,6 @@ class StaticFeatureExtractor:
     This class is not instantiated directly; it is the base class for other implementations.
     """
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, hashes: SampleHashes):
         #
         # note: a subclass should define ctor parameters for its own use.
@@ -359,7 +357,7 @@ class CallHandle:
     inner: Any
 
 
-class DynamicFeatureExtractor:
+class DynamicFeatureExtractor(abc.ABC):
     """
     DynamicFeatureExtractor defines the interface for fetching features from a
     sandbox' analysis of a sample; extractors that rely on statically analyzing
@@ -372,8 +370,6 @@ class DynamicFeatureExtractor:
     This class is not instantiated directly; it is the base class for other implementations.
     """
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, hashes: SampleHashes):
         #
         # note: a subclass should define ctor parameters for its own use.