gh-104263: Rely on Py_NAN and introduce Py_INFINITY (GH-104202)

This PR removes `_Py_dg_stdnan` and `_Py_dg_infinity` in favour of
using the standard `NAN` and `INFINITY` macros provided by C99.
This change has the side-effect of fixing a bug on MIPS where the
hard-coded value used by `_Py_dg_stdnan` gave a signalling NaN
rather than a quiet NaN.
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Co-authored-by: Mark Dickinson <dickinsm@gmail.com>