Update patch status

* d/p/{debian/,}Use-Python-3-for-tests.patch:
  Mark patch as forwarded. It was previously considered
  Debian-specific, but upstream seem interested.
* Mark all other patches as applied upstream for 2018.3

debian/patches/series

@@ -2,4 +2,4 @@ test-concurrency-Use-Python-3-syntax-for-octal.patch
 test-concurrency-Replace-range-with-xrange.patch
 test-concurrency-Explicitly-use-floor-division.patch
 tests-bootloader-entries-crosscheck-Use-Python-3-friendly.patch
-debian/Use-Python-3-for-tests.patch
+Use-Python-3-for-tests.patch