Recognize classes that explicitly inherit from `ABC` or define `metaclass=ABCMeta` as abstract (#3446)

Related to #179 and #3098. Tweaks `utils.class_is_abstract` to not
depend purely on the presence of abstract methods, and instead also
return True for classes that either explicitly inherit from `abc.ABC`,
or explicitly define `abc.ABCMeta` as a metaclass.

This means that classes like:

    class Foo(AbstractParent, ABC):
        ...

    class Bar(AbstractParent, metaclass=ABCMeta):
        ...

no longer trigger W0223.

2 files changed, 35 insertions, 16 deletions

diff --git a/tests/functional/a/abstract_method_py3.py b/tests/functional/a/abstract_method_py3.py
index f4e3dd568..8bb491819 100644
--- a/tests/functional/a/abstract_method_py3.py
+++ b/tests/functional/a/abstract_method_py3.py
@@ -5,6 +5,7 @@ from __future__ import print_function
 # pylint: disable=too-few-public-methods, useless-object-inheritance
 import abc
 
+
 class Abstract(object):
     def aaaa(self):
         """should be overridden in concrete class"""
@@ -25,6 +26,24 @@ class AbstractB(Abstract):
         """should be overridden in concrete class"""
         raise NotImplementedError()
 
+class AbstractC(AbstractB, abc.ABC):
+    """
+    Abstract class.
+
+    Should not trigger a warning for unimplemented
+    abstract methods, because of explicit abc.ABC inheritance.
+    """
+
+
+class AbstractD(AbstractB, metaclass=abc.ABCMeta):
+    """
+    Abstract class.
+
+    Should not trigger a warning for unimplemented
+    abstract methods, because of explicit metaclass.
+    """
+
+
 class Concrete(Abstract): # [abstract-method]
     """Concrete class"""
 
diff --git a/tests/functional/a/abstract_method_py3.txt b/tests/functional/a/abstract_method_py3.txt
index 17f8abf4e..73c97d8be 100644
--- a/tests/functional/a/abstract_method_py3.txt
+++ b/tests/functional/a/abstract_method_py3.txt
@@ -1,16 +1,16 @@
-abstract-method:28:Concrete:"Method 'bbbb' is abstract in class 'Abstract' but is not overridden"

-abstract-method:51:Container:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

-abstract-method:51:Container:"Method '__iter__' is abstract in class 'Structure' but is not overridden"

-abstract-method:51:Container:"Method '__len__' is abstract in class 'Structure' but is not overridden"

-abstract-method:57:Sizable:"Method '__contains__' is abstract in class 'Structure' but is not overridden"

-abstract-method:57:Sizable:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

-abstract-method:57:Sizable:"Method '__iter__' is abstract in class 'Structure' but is not overridden"

-abstract-method:63:Hashable:"Method '__contains__' is abstract in class 'Structure' but is not overridden"

-abstract-method:63:Hashable:"Method '__iter__' is abstract in class 'Structure' but is not overridden"

-abstract-method:63:Hashable:"Method '__len__' is abstract in class 'Structure' but is not overridden"

-abstract-method:68:Iterator:"Method '__contains__' is abstract in class 'Structure' but is not overridden"

-abstract-method:68:Iterator:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

-abstract-method:68:Iterator:"Method '__len__' is abstract in class 'Structure' but is not overridden"

-abstract-method:87:BadComplexMro:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

-abstract-method:87:BadComplexMro:"Method '__len__' is abstract in class 'AbstractSizable' but is not overridden"

-abstract-method:87:BadComplexMro:"Method 'length' is abstract in class 'AbstractSizable' but is not overridden"

+abstract-method:47:Concrete:"Method 'bbbb' is abstract in class 'Abstract' but is not overridden"

+abstract-method:70:Container:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

+abstract-method:70:Container:"Method '__iter__' is abstract in class 'Structure' but is not overridden"

+abstract-method:70:Container:"Method '__len__' is abstract in class 'Structure' but is not overridden"

+abstract-method:76:Sizable:"Method '__contains__' is abstract in class 'Structure' but is not overridden"

+abstract-method:76:Sizable:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

+abstract-method:76:Sizable:"Method '__iter__' is abstract in class 'Structure' but is not overridden"

+abstract-method:82:Hashable:"Method '__contains__' is abstract in class 'Structure' but is not overridden"

+abstract-method:82:Hashable:"Method '__iter__' is abstract in class 'Structure' but is not overridden"

+abstract-method:82:Hashable:"Method '__len__' is abstract in class 'Structure' but is not overridden"

+abstract-method:87:Iterator:"Method '__contains__' is abstract in class 'Structure' but is not overridden"

+abstract-method:87:Iterator:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

+abstract-method:87:Iterator:"Method '__len__' is abstract in class 'Structure' but is not overridden"

+abstract-method:106:BadComplexMro:"Method '__hash__' is abstract in class 'Structure' but is not overridden"

+abstract-method:106:BadComplexMro:"Method '__len__' is abstract in class 'AbstractSizable' but is not overridden"

+abstract-method:106:BadComplexMro:"Method 'length' is abstract in class 'AbstractSizable' but is not overridden"