Issue #767645: fix os.path.supports_unicode_filenames definition

os.listdir(str) always returns unicode and it can return non-ascii filenames
even if supports_unicode_filenames is False.

Doc/library/os.path.rst

@@ -320,5 +320,4 @@ applications should use string objects to access all files.
 .. data:: supports_unicode_filenames
 
    True if arbitrary Unicode strings can be used as file names (within limitations
-   imposed by the file system), and if :func:`os.listdir` returns strings that
-   contain characters that cannot be represented by ASCII.
+   imposed by the file system).