python2Packages.pycairo: backport test fix

Fix pytest failure of `test_svg_surface_get_document_unit`.

This fix is backported to python 2 from upstream. Upstream has dropped
support for python 2, but the fix works regardless of python versions.
Hence we backport the fix for python 2, since many nix packages still
depend on python2Packages.pycairo.

Upstream: https://github.com/pygobject/pycairo/commit/678edd94d8a6dfb5d51f9c3549e6ee8c90a73744
Co-authored-by: Brian Leung <29217594+leungbk@users.noreply.github.com>

pkgs/development/python2-modules/pycairo/default.nix

@@ -1,5 +1,6 @@
 { lib
 , fetchFromGitHub
+, fetchpatch
 , meson
 , ninja
 , buildPythonPackage
@@ -22,6 +23,13 @@ buildPythonPackage rec {
     sha256 = "142145a2whvlk92jijrbf3i2bqrzmspwpysj0bfypw0krzi0aa6j";
   };
 
+  patches = [
+    (fetchpatch {
+      url = "https://github.com/pygobject/pycairo/commit/678edd94d8a6dfb5d51f9c3549e6ee8c90a73744.patch";
+      sha256 = "sha256-HmP69tUGYxZvJ/M9FJHwHTCjb9Kf4aWRyMT4wSymrT0=";
+    })
+  ];
+
   nativeBuildInputs = [
     meson
     ninja