Convert the Generic/ABC metaclass hack into a generic solution

Fixes gh-1092

Author: njsmith

trio/_channel.py

@@ -102,21 +102,8 @@ def statistics(self):
         )
 
 
-try:
-    from typing import GenericMeta
-except ImportError:
-    from abc import ABCMeta
-
-    class _GenericNoPublicConstructor(NoPublicConstructor, ABCMeta):
-        pass
-else:
-
-    class _GenericNoPublicConstructor(NoPublicConstructor, GenericMeta):
-        pass
-
-
 @attr.s(cmp=False, repr=False)
-class MemorySendChannel(SendChannel, metaclass=_GenericNoPublicConstructor):
+class MemorySendChannel(SendChannel, metaclass=NoPublicConstructor):
     _state = attr.ib()
     _closed = attr.ib(default=False)
     # This is just the tasks waiting on *this* object. As compared to
@@ -234,9 +221,7 @@ async def aclose(self):
 
 
 @attr.s(cmp=False, repr=False)
-class MemoryReceiveChannel(
-    ReceiveChannel, metaclass=_GenericNoPublicConstructor
-):
+class MemoryReceiveChannel(ReceiveChannel, metaclass=NoPublicConstructor):
     _state = attr.ib()
     _closed = attr.ib(default=False)
     _tasks = attr.ib(factory=set)

trio/_util.py

@@ -257,7 +257,21 @@ def __getitem__(self, _):
         return self
 
 
-class Final(type):
+# If a new class inherits from any ABC, then the new class's metaclass has to
+# inherit from ABCMeta. If a new class inherits from typing.Generic, and
+# you're using Python 3.6 or earlier, then the new class's metaclass has to
+# inherit from typing.GenericMeta. Some of the classes that want to use Final
+# or NoPublicConstructor inherit from ABCs and generics, so Final has to
+# inherit from these metaclasses. Fortunately, GenericMeta inherits from
+# ABCMeta, so inheriting from GenericMeta alone is sufficient (when it
+# exists at all).
+if hasattr(t, "GenericMeta"):
+    BaseMeta = t.GenericMeta
+else:
+    BaseMeta = ABCMeta
+
+
+class Final(BaseMeta):
     """Metaclass that enforces a class to be final (i.e., subclass not allowed).
 
     If a class uses this metaclass like this::