bpo-45412: Add _PY_SHORT_FLOAT_REPR macro (GH-31171)

Remove the HAVE_PY_SET_53BIT_PRECISION macro (moved to the internal
C API).

* Move HAVE_PY_SET_53BIT_PRECISION macro to pycore_pymath.h.
* Replace PY_NO_SHORT_FLOAT_REPR macro with _PY_SHORT_FLOAT_REPR
  macro which is always defined. gcc -Wundef emits a warning when
  using _PY_SHORT_FLOAT_REPR but the macro is not defined, if
  pycore_pymath.h include was forgotten.