Issue #21127: Path objects can now be instantiated from str subclass instances (such as numpy.str_).

Thanks to Antony Lee for the report and preliminary patch.

Lib/pathlib.py

@@ -574,8 +574,8 @@ class PurePath(object):
             if isinstance(a, PurePath):
                 parts += a._parts
             elif isinstance(a, str):
-                # Assuming a str
-                parts.append(a)
+                # Force-cast str subclasses to str (issue #21127)
+                parts.append(str(a))
             else:
                 raise TypeError(
                     "argument should be a path or str object, not %r"