python.pkgs.doc8: add pygments dependency

Without pygments doc8 will throw "errors" since it cannot check code
blocks. Pygments has since been added to the dependencies upstream [1],
though that hasn't made it into a release yet. I think we should already
adopt this now.

[1] https://github.com/PyCQA/doc8/commit/b6931816585e224019afddfc9d78ecbcf0549c74

pkgs/development/python-modules/doc8/default.nix

@@ -7,6 +7,7 @@
 , chardet
 , stevedore
 , restructuredtext_lint
+, pygments
 }:
 
 buildPythonPackage rec {
@@ -19,7 +20,14 @@ buildPythonPackage rec {
   };
 
   buildInputs = [ pbr ];
-  propagatedBuildInputs = [ docutils six chardet stevedore restructuredtext_lint ];
+  propagatedBuildInputs = [
+    docutils
+    six
+    chardet
+    stevedore
+    restructuredtext_lint
+    pygments
+  ];
 
   doCheck = false;