python36Packages.black: fix broken dependency

In PR #96456, black was upgraded from 19.10b0 to 20.8b1.  This new version of
black depends on the dataclasses module, but that was only introduced into the
standard library in Python 3.7; earlier versions of Python require the backport
package of the same name.  This commit addresses the missing dependency.

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

@@ -5,6 +5,7 @@
 , appdirs
 , attrs
 , click
+, dataclasses
 , mypy-extensions
 , pathspec
 , regex
@@ -59,7 +60,7 @@ buildPythonPackage rec {
     toml
     typed-ast
     typing-extensions
-  ];
+  ] ++ lib.optional (pythonOlder "3.7") dataclasses;
 
   meta = with lib; {
     description = "The uncompromising Python code formatter";