Implement all remaining Path issues and use it instead of strings

jedi/inference/gradual/utils.py

@@ -1,4 +1,5 @@
 import os
+from pathlib import Path
 
 from jedi.inference.gradual.typeshed import TYPESHED_PATH, create_stub_module
 
@@ -9,14 +10,18 @@ def load_proper_stub_module(inference_state, file_io, import_names, module_node)
     module.
     """
     path = file_io.path
-    assert path.endswith('.pyi')
-    if path.startswith(TYPESHED_PATH):
-        # /foo/stdlib/3/os/__init__.pyi -> stdlib/3/os/__init__
-        rest = path[len(TYPESHED_PATH) + 1: -4]
-        split_paths = tuple(rest.split(os.path.sep))
+    path = Path(path)
+    assert path.suffix == '.pyi'
+    try:
+        relative_path = path.relative_to(TYPESHED_PATH)
+    except ValueError:
+        pass
+    else:
+        # /[...]/stdlib/3/os/__init__.pyi -> stdlib/3/os/__init__
+        rest = relative_path.with_suffix('')
         # Remove the stdlib/3 or third_party/3.6 part
-        import_names = split_paths[2:]
-        if import_names[-1] == '__init__':
+        import_names = rest.parts[2:]
+        if rest.name == '__init__':
             import_names = import_names[:-1]
 
     if import_names is not None: