> Why wouldn't you give byte variables in env too?

The problem with the canonicalization to bytes is to choice of the preferred type. Eg. env={'PATH': 'a', b'PATH': b'b'}: should we use 'a', 'b' or raise an error?

subprocess does already convert environ keys and values to bytes on Unix (in subprocess._execute_child() if _posixsubprocess module is present, and in posix_execve()).

os.get_exec_path() should also support b'PATH' key.