Simplify Traversable signature (#10934)

Simplify Traversable.open() signature. This is necessary so that implentors can
reasonanbly implement this method. For example `zipfile.Path.open()` (which
is considered a `Traversable`) only supports this subset.

Make `Traversable.__truediv__` and `joinpath` arguments pos-only. The
arguments are named differently in both `pathlib.Path` and `zipfile.Path`.

tests/stubtest_allowlists/py310.txt

@@ -152,6 +152,10 @@ typing.ParamSpec(Args|Kwargs).__origin__
 # https://github.com/python/mypy/issues/15302
 typing.NewType.__call__
 
+# Problematic protocol signatures at runtime, see source code comments.
+importlib.abc.Traversable.joinpath
+importlib.abc.Traversable.open
+
 # Super-special typing primitives
 typing\.NamedTuple
 typing\.Annotated

tests/stubtest_allowlists/py311.txt

@@ -107,6 +107,10 @@ platform.uname_result.processor
 unittest.TestCase.__init_subclass__
 unittest.case.TestCase.__init_subclass__
 
+# Problematic protocol signature at runtime, see source code comments.
+importlib.abc.Traversable.open
+importlib.resources.abc.Traversable.open
+
 # Super-special typing primitives
 typing\._SpecialForm.*
 typing\.NamedTuple

tests/stubtest_allowlists/py312.txt

@@ -96,6 +96,10 @@ platform.uname_result.processor
 unittest.TestCase.__init_subclass__
 unittest.case.TestCase.__init_subclass__
 
+# Problematic protocol signature at runtime, see source code comments.
+importlib.abc.Traversable.open
+importlib.resources.abc.Traversable.open
+
 # Super-special typing primitives
 typing\._SpecialForm.*
 typing\.NamedTuple

tests/stubtest_allowlists/py39.txt

@@ -141,6 +141,9 @@ types.MemberDescriptorType.__get__
 types.MethodDescriptorType.__get__
 types.WrapperDescriptorType.__get__
 
+# Problematic protocol signature at runtime, see source code comments.
+importlib.abc.Traversable.open
+
 # Super-special typing primitives
 typing\.NamedTuple
 typing\.Annotated