pricehist: fix build

Fix build for the newer version of lxml provided by nixpkgs

pkgs/tools/misc/pricehist/default.nix

@@ -9,6 +9,7 @@
 , pytest-mock
 , responses
 , pytestCheckHook
+, pythonRelaxDepsHook
 }:
 
 buildPythonApplication rec {
@@ -31,11 +32,20 @@ buildPythonApplication rec {
     poetry-core
   ];
 
+  nativeBuildInputs = [
+    pythonRelaxDepsHook
+  ];
+
   nativeCheckInputs = [
     responses
     pytest-mock
     pytestCheckHook
   ];
+
+  pythonRelaxDeps = [
+    # https://gitlab.com/chrisberkhout/pricehist/-/issues/6
+    "lxml"
+  ];
   meta = with lib; {
     description = "A command-line tool for fetching and formatting historical price data, with support for multiple data sources and output formats";
     homepage = "https://gitlab.com/chrisberkhout/pricehist";