See #109. Eventually, this could be used to support [gmpy2-like rounding modes](https://gmpy2.readthedocs.io/en/latest/mpfr.html#gmpy2.mpfr.__format__) in ``mpf/mpc.__format__()``.
See #109. Eventually, this could be used to support gmpy2-like rounding modes in
mpf/mpc.__format__().