fierce: fix build

because we're playing with its install requirements, add a
pythonImportsCheck to give us a chance to discover brokenness. technically
this isn't a realistic test of end user behaviour as this is really an
application, not a python module, but it seems to have a pretty stable
import name of `fierce`, so it works.

pkgs/tools/security/fierce/default.nix

@@ -11,8 +11,16 @@ python3.pkgs.buildPythonApplication rec {
     sha256 = "11yaz8ap9swx95j3wpqh0b6jhw6spqgfnsyn1liw9zqi4jwgiax7";
   };
 
+  postPatch = ''
+    substituteInPlace requirements.txt --replace 'dnspython==1.16.0' 'dnspython'
+  '';
+
   propagatedBuildInputs = [ python3.pkgs.dns ];
 
+  # tests require network access
+  doCheck = false;
+  pythonImportsCheck = [ "fierce" ];
+
   meta = with stdenv.lib; {
     homepage = "https://github.com/mschwager/fierce";
     description = "DNS reconnaissance tool for locating non-contiguous IP space";