bpo-32539: Fix OSError for os.listdir() for extended-length paths on Windows (#5169)

See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx?f=255&MSPPError=-2147217396#maxpath

Paths that begin with `\\?\` are "extended-length paths".
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index c9886d5..cecbb45 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -2385,7 +2385,7 @@
     if (len > 0) {
         char ch = namebuf[len-1];
         if (ch != SEP && ch != ALTSEP && ch != ':')
-            namebuf[len++] = '/';
+            namebuf[len++] = SEP;
         strcpy(namebuf + len, "*.*");
     }