Add a comment for virtualenv

Author: mayeut

tests/requirements.txt

@@ -8,6 +8,8 @@ pytest-rerunfailures
 pytest-xdist
 scripttest
 setuptools
+# macOS (darwin) arm64 always uses virtualenv >= 20.0
+# for other platforms, it depends on python version
 virtualenv < 20.0 ; python_version < '3.10' and (sys_platform != 'darwin' or platform_machine != 'arm64')
 virtualenv >= 20.0 ; python_version >= '3.10' or (sys_platform == 'darwin' and platform_machine == 'arm64')
 werkzeug