style: deal with B024 added to flake8 recently

More details about B024 here PyCQA/flake8-bugbear#274

* ./skywalking/agent/protocol/__init__.py:22:1: B024 Protocol is an abstract base class, but it has no abstract methods.
Set 'heartbeat', 'report' and 'report_log' as abstract method

* ./skywalking/trace/span.py:36:1: B024 Span is an abstract base class, but it has no abstract methods.
Prevent base class Span from being directly instantiated by raising error in __new__ with reference to
https://stackoverflow.com/questions/7989042/preventing-a-class-from-direct-instantiation-in-python

* ./tests/plugin/base.py:35:1: B024 TestPluginBase is an abstract base class
Due to pytest may instantiate parent class for searching test functions, TestPluginBase has to be instantiable.

Author: jiang1997

skywalking/agent/__init__.py

@@ -35,7 +35,7 @@
     from skywalking.trace.context import Segment
 
 __started = False
-__protocol = Protocol()  # type: Protocol
+__protocol = None  # type: Protocol
 __heartbeat_thread = __report_thread = __log_report_thread = __query_profile_thread = __command_dispatch_thread \
     = __send_profile_thread = __queue = __log_queue = __snapshot_queue = __finished = None
 

skywalking/agent/protocol/__init__.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-from abc import ABC
+from abc import ABC, abstractmethod
 from queue import Queue
 
 
@@ -29,12 +29,15 @@ def fork_after_in_parent(self):
     def fork_after_in_child(self):
         pass
 
+    @abstractmethod
     def heartbeat(self):
         raise NotImplementedError()
 
+    @abstractmethod
     def report(self, queue: Queue, block: bool = True):
         raise NotImplementedError()
 
+    @abstractmethod
     def report_log(self, queue: Queue, block: bool = True):
         raise NotImplementedError()
 

skywalking/trace/span.py

@@ -16,7 +16,6 @@
 #
 
 import time
-from abc import ABC
 from collections import defaultdict
 from typing import List, Union, DefaultDict
 from typing import TYPE_CHECKING
@@ -33,7 +32,12 @@
 
 
 @tostring
-class Span(ABC):
+class Span:
+    def __new__(cls, *args, **kwargs):
+        if cls is Span:
+            raise TypeError(f"only children of '{cls.__name__}' may be instantiated")
+        return object.__new__(cls, *args, **kwargs) 
+    
     def __init__(
             self,
             context: 'SpanContext',

tests/plugin/base.py

@@ -18,7 +18,6 @@
 import inspect
 import os
 import sys
-from abc import ABC
 from difflib import Differ
 from os.path import dirname
 
@@ -32,7 +31,7 @@
     from yaml import SafeLoader as Loader
 
 
-class TestPluginBase(ABC):
+class TestPluginBase:
     def validate(self, expected_file_name=None):
         # type: (str) -> Response