pythonPackages.sip_5: add `platform_tag` attribute

It's value is platform dependend and needs to be specified in `pyproject.toml` for modules that use sip_5. Instead of defining it for each module, we can add it to sip_5, so it can be reused.

pkgs/applications/misc/calibre/default.nix

@@ -1,5 +1,4 @@
 { lib
-, stdenv
 , mkDerivation
 , fetchurl
 , poppler_utils
@@ -42,18 +41,11 @@ mkDerivation rec {
   ++ lib.optional (!unrarSupport) ./dont_build_unrar_plugin.patch;
 
   escaped_pyqt5_dir = builtins.replaceStrings ["/"] ["\\/"] (toString python3Packages.pyqt5);
-  platform_tag =
-    if stdenv.hostPlatform.isDarwin then
-      "WS_MACX"
-    else if stdenv.hostPlatform.isWindows then
-      "WS_WIN"
-    else
-      "WS_X11";
 
   prePatch = ''
     sed -i "s/\[tool.sip.project\]/[tool.sip.project]\nsip-include-dirs = [\"${escaped_pyqt5_dir}\/share\/sip\/PyQt5\"]/g" \
       setup/build.py
-    sed -i "s/\[tool.sip.bindings.pictureflow\]/[tool.sip.bindings.pictureflow]\ntags = [\"${platform_tag}\"]/g" \
+    sed -i "s/\[tool.sip.bindings.pictureflow\]/[tool.sip.bindings.pictureflow]\ntags = [\"${python3Packages.sip_5.platform_tag}\"]/g" \
       setup/build.py
 
     # Remove unneeded files and libs

pkgs/development/python-modules/sip/5.x.nix

@@ -1,4 +1,4 @@
-{ lib, fetchPypi, buildPythonPackage, packaging, toml }:
+{ lib, stdenv, fetchPypi, buildPythonPackage, packaging, toml }:
 
 buildPythonPackage rec {
   pname = "sip";
@@ -17,6 +17,20 @@ buildPythonPackage rec {
 
   pythonImportsCheck = [ "sipbuild" ];
 
+  # FIXME: Why isn't this detected automatically?
+  # Needs to be specified in pyproject.toml, e.g.:
+  # [tool.sip.bindings.MODULE]
+  # tags = [PLATFORM_TAG]
+  platform_tag =
+    if stdenv.targetPlatform.isLinux then
+      "WS_X11"
+    else if stdenv.targetPlatform.isDarwin then
+      "WS_MACX"
+    else if stdenv.targetPlatform.isWindows then
+      "WS_WIN"
+    else
+      throw "unsupported platform";
+
   meta = with lib; {
     description = "Creates C++ bindings for Python modules";
     homepage    = "http://www.riverbankcomputing.co.uk/";