python3Packages.pythonocc-core: fix build

The `opencascade-occt` and `python3Packages.pythonocc-core` versions
need to match. `opencascade-occt` was upgraded in
https://github.com/NixOS/nixpkgs/pull/309347 but the corresponding
version of `pythonocc-core` is not released yet. Also, the
corresponding version of `pythonocc-core` will need at least swig
4.1.1 to build.

For now, just build `pythonocc-core` with the previous version of
`opencascade-occt`, which luckily is also still in nixpkgs.

Fixes https://github.com/NixOS/nixpkgs/issues/312955

pkgs/top-level/python-packages.nix

@@ -12494,6 +12494,7 @@ self: super: with self; {
     inherit (pkgs) fontconfig rapidjson;
     inherit (pkgs.xorg) libX11 libXi libXmu libXext;
     inherit (pkgs.darwin.apple_sdk.frameworks) Cocoa;
+    opencascade-occt = pkgs.opencascade-occt_7_6;
   });
 
   python-olm = callPackage ../development/python-modules/python-olm { };