Currently __trunc__ is used for two purposes:

* In math.trunc() which just calls __trunc__.
* In int() and PyNumber_Long() as a fallback if neither __int__ nor
__index__ are implemented.

Unlike to __int__ and __index__ there is not a slot corresponding to
__trunc__. So using it is slower than using __int__ or __index__. float
and Decimal implement __int__ and __trunc__ by the same code. Only
Fraction implements __trunc__ but not __int__.

I propose to deprecate the falling back to __trunc__ for converting to
int and remove it in future. Fraction should implement __int__.

We cannot use __trunc__ for setting the nb_int slot because it can
return non-int.
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