CI updates: add Python 3.10 and drop Python 3.8 support

[hwwhww]

Issue

For efficiency and using more new built-in libraries, we can consider dropping Python 3.8 support now.

[hwwhww]

Note that it may affect the eth2spec Python package users if we start to write code that Python 3.8 doesn't support.

[ChihChengLiang]

I'm also preparing an update on future python versions https://github.com/ChihChengLiang/milagro_bls_binding/issues/34

[djrtwo]

When we move from 3.8 to 3.10, we should build test vectors with both and ensure they have the same output

[ChihChengLiang]

@hwwhww @protolambda I have the milagro_bls_binding 1.8.0 ready. It works with my macos/M1/python3.10 setting.