opencv: Work around build failure with enableContrib & Python

pkgs/development/libraries/opencv/3.x.nix

@@ -90,9 +90,14 @@ stdenv.mkDerivation rec {
       done
     '');
 
-  # This prevents cmake from using libraries in impure paths (which causes build failure on non NixOS)
+  # This prevents cmake from using libraries in impure paths (which
+  # causes build failure on non NixOS)
+  # Also, work around https://github.com/NixOS/nixpkgs/issues/26304 with
+  # what appears to be some stray headers in dnn/misc/tensorflow
+  # in contrib when generating the Python bindings:
   postPatch = ''
     sed -i '/Add these standard paths to the search paths for FIND_LIBRARY/,/^\s*$/{d}' CMakeLists.txt
+    sed -i -e 's|if len(decls) == 0:|if len(decls) == 0 or "opencv2/" not in hdr:|' ./modules/python/src2/gen2.py
   '';
 
   preConfigure =