format() should take into account rounding mode of the context?

[skirpichev]

Reproducer:

>>> from gmpy2 import *
>>> format(mpfr(2.675), '.2f')
'2.67'
>>> get_context().round = RoundUp
>>> format(mpfr(2.675), '.2f')
'2.67'

I'm not sure whether it's a bug or a feature.

On one hand, changing rounding direction does affect C printf():

$ cat a.c
#include <fenv.h>
#include <stdio.h>
int main(void)
{
    double x = 2.675;
    fesetround(FE_TONEAREST);
    printf("%.2f\n", x);
    fesetround(FE_UPWARD);
    printf("%.2f\n", x);
    return 0;
}
$ gcc a.c -std=c11 -Wpedantic -lm && ./a.out
2.67
2.68

On another hand, current behaviour is consistent with CPython, which just does round to nearest (with ties to even), unless it's configured to use libc's printf (I'm not sure if this option is still available).