Add __fspath__ to pathlib (#1774)

Author: slozier

Src/StdLib/Lib/pathlib.py

@@ -648,6 +648,10 @@ def __str__(self):
                                                   self._parts) or '.'
             return self._str
 
+    # ironpython: backport from 3.6
+    def __fspath__(self):
+        return str(self)
+
     def as_posix(self):
         """Return the string representation of the path with forward (/)
         slashes."""