haskell.compiler.ghc8104: patch for sphinx >= 4.0

With sphinx 4, interpreting the conf.py fails due to a decode
error: https://gitlab.haskell.org/ghc/ghc/-/issues/19962

The fix is an one line change which we have to backport from GHC master.
Thus ghcHEAD is not affected by this problem.

9.2 and 8.10.6 will most likely have a fix for this.

TODO: Patch 8.8.4 and 9.0.1

pkgs/development/compilers/ghc/8.10.4.nix

@@ -125,6 +125,9 @@ stdenv.mkDerivation (rec {
     # upstream patch. Don't forget to check backport status of the upstream patch
     # when adding new GHC releases in nixpkgs.
     ./respect-ar-path.patch
+    # Fix documentation configuration which causes a syntax error with sphinx 4.*
+    # See https://gitlab.haskell.org/ghc/ghc/-/issues/19962, remove at 8.10.6.
+    ./sphinx-4-configuration.patch
   ] ++ lib.optionals stdenv.isDarwin [
     # Make Block.h compile with c++ compilers. Remove with the next release
     (fetchpatch {

pkgs/development/compilers/ghc/sphinx-4-configuration.patch

@@ -0,0 +1,11 @@
+--- ghc-8.10.4/docs/users_guide/conf.py.orig	2021-06-21 13:46:34.196383559 +0200
++++ ghc-8.10.4/docs/users_guide/conf.py	2021-06-21 13:46:54.839349941 +0200
+@@ -100,7 +100,7 @@
+ latex_elements = {
+     'inputenc': '',
+     'utf8extra': '',
+-    'preamble': '''
++    'preamble': r'''
+ \usepackage{fontspec}
+ \usepackage{makeidx}
+ \setsansfont{DejaVu Sans}