Use of deprecated `typing` aliases

Betterproto currently is using some `typing` imports that are now deprecated, though they are noted in the Python docs to be guaranteed to remain until at least 3.14.

For more details, see [this Python docs section](https://docs.python.org/3.11/library/typing.html#deprecated-aliases). Betterproto uses some of these such as `typing.Generator` within `src/betterproto/__init__.py`.