requirements: install orjson on non-win

orjson doesn't provide win32 (32-bit) python wheels for windows, only 64-bit (win_amd64).

build from source takes total CI time to 5 min (from 2) so no good.

I tried to only install 64-bit wheels but environment markers don't let me select what I want.

requirements.txt

@@ -12,3 +12,10 @@ ttfautohint-py==0.5.1
 
 # optional, for experimental reading/writing ufoLib2's UFO as json
 cattrs==23.1.2
+# orjson currently doesn't ship 32-bit wheels for Windows
+# https://github.com/ijl/orjson/issues/409
+# platform_machine environment marker returns 'AMD64' on Windows even if Python is 32-bit
+# so there's no way to tell pip to install orjson only on 64-bit Win Python; hence
+# don't install orjson on Windows (works fine without it using stdlib json module)
+# https://stackoverflow.com/a/75411662
+orjson==3.9.2; platform_python_implementation == 'CPython' and platform_system != 'Windows'