ffi: expose PyDateTime_*_GET_TZINFO on PyPy

[exg]

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[bschoenmaeckers]

Can we use this to simplify PyTzInfoAccess?

[bschoenmaeckers]

Seems like PyTzInfoAccess cannot be simplified on 3.9 because the ffi function is for 3.10 onwards .

[bschoenmaeckers]

I think we can conditional compile on the Py_3_10 flag in PyTzInfoAccess.

[exg]

@davidhewitt do you plan to release a new version with this change anytime soon?

[davidhewitt]

#5107