Minor updates to protocol semantics

[ilevkivskyi]

This updates protocol semantic according to the discussion in python/typing#464:

• None should be considered a subtype of an empty protocol
• method = None rule should apply only to callable protocol members
• issublcass() is prohibited for protocols with non-method members.

Fixes #3906
Fixes #3938
Fixes #3939

[gvanrossum]

@JukkaL Can you review this?

@ilevkivskyi What changed so that reveal_type() of builtins.list now comes out as builtins.list[Any]?

[ilevkivskyi]

What changed so that reveal_type() of builtins.list now comes out as builtins.list[Any]?

This:

 def visit_type_type(self, t: TypeType) -> None:
-    pass
+    t.item.accept(self)

This fix will anyway happen in #3952, but without it this test (expectedly) crashed.

[JukkaL]

Thanks for the PR! Looks good, just a few minor comments.

[JukkaL]

Also test that None is compatible with a non-empty protocol when not using strict optional checking.

[JukkaL]

Move the body to a separate method, as this function is already quite long. Maybe also move the body of the above if statement (from line 269) to a separate method. You could also move the if statement to be nested under the above if statement (line 267). You'd only need the check if e.callee.fullname == 'builtins.subclass' which would simplify things slightly.

[JukkaL]

Can you also fix the merge conflict?

[ilevkivskyi]

Thanks! I addressed CR and fixed the conflict.