weechat: re-enable docs

The doc build was failing because of an unpatched `/usr/bin/env` shebang
in the `pygments.rb` gem, and weechat uses asciidoctor with the pygments
highlighter. Adding `python3` to `pygments.rb`'s `buildInputs` fixes it.

pkgs/applications/networking/irc/weechat/default.nix

@@ -50,7 +50,7 @@ let
 
       cmakeFlags = with lib; [
         "-DENABLE_MAN=ON"
-        "-DENABLE_DOC=OFF"         # TODO(@ncfavier): Documentation fails to build, was deactivated to push through security update
+        "-DENABLE_DOC=ON"
         "-DENABLE_TESTS=${if enableTests then "ON" else "OFF"}"
       ]
         ++ optionals stdenv.isDarwin ["-DICONV_LIBRARY=${libiconv}/lib/libiconv.dylib"]

pkgs/development/ruby-modules/gem-config/default.nix

@@ -545,6 +545,10 @@ in
     buildInputs = [ openssl ];
   };
 
+  "pygments.rb" = attrs: {
+    buildInputs = [ python3 ];
+  };
+
   rack = attrs: {
     meta.mainProgram = "rackup";
   };