Add PyOS_double_to_string flag macros
These flags are defined in cpython in https://github.com/python/cpython/blob/63298930fb531ba2bb4f23bc3b915dbf1e17e9e1/Include/pystrtod.h#L30-L34
They are input for PyOS_double_to_string
which is implemented in pypy at https://foss.heptapod.net/pypy/pypy/-/blob/branch/default/pypy/module/cpyext/pystrtod.py#L122