Patch doctest-0.20.0 to accommodate Backend becoming abstract

See ghc@4aa3c5bd

patches/doctest-0.20.0.patch

@@ -71,3 +71,18 @@ index ac4c9be..1669692 100644
 +lhsDocString :: SrcSpan -> HsDocString -> LHsDocString
 +lhsDocString = L
 +#endif
+diff --git a/src/GhcUtil.hs b/src/GhcUtil.hs
+index de36558..a683027 100644
+--- a/src/GhcUtil.hs
++++ b/src/GhcUtil.hs
+@@ -103,7 +103,9 @@ setHaddockMode dynflags = (dopt_set dynflags Opt_Haddock) {
+ #else
+ setHaddockMode dynflags = (gopt_set dynflags Opt_Haddock) {
+ #endif
+-#if __GLASGOW_HASKELL__ >= 901
++#if __GLASGOW_HASKELL__ >= 903
++      backend   = noBackend
++#elif __GLASGOW_HASKELL__ >= 901
+       backend   = NoBackend
+ #else
+       hscTarget = HscNothing