Add __index__ fallback to mpz constructor (fixes #237)

[WebVishnu]

Add index fallback to mpz constructor (fixes #237)

Problem

The mpz constructor only supports objects with __int__() method, failing with objects that only implement __index__().

Solution

Added fallback mechanism that:

1. Tries __int__() first (maintains backward compatibility)
2. Falls back to __index__() if __int__() not available

Changes

• Modified new_impl function in main.c to add __index__() fallback

[skirpichev]

This also needs tests.

[WebVishnu]

@skirpichev should I create a new file of test cases for this update?

[skirpichev]

should I create a new file of test cases for this update?

No, you could add tests like for a case with __int__() dunder. See test_mpz_interface().

[skirpichev]

LGTM

[skirpichev]

Thanks.