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`.

test_cases/stdlib/check_importlib.py

@@ -0,0 +1,13 @@
+import importlib.abc
+import pathlib
+import sys
+import zipfile
+
+# Assert that some Path classes are Traversable.
+if sys.version_info >= (3, 9):
+
+    def traverse(t: importlib.abc.Traversable) -> None:
+        pass
+
+    traverse(pathlib.Path())
+    traverse(zipfile.Path(""))