https://github.com/python-hyper/hyper-h2/blob/3b0b241d79f5a9ff9382bbc038f84862e0d80abf/src/h2/utilities.py#L20-L26.

Hi, when a python process runs with a flag -bb, the above part of code will throw exception and make h2 not work. May I ask why we define both bytes and string in the frozenset? Is it possible to use only bytes or string? Frozenset will compare keys for deduplication.