Use the result of __index__ when converting to Rust BigInts.

[noam93k]

Thank you for contributing to pyo3!

Here are some things you should check for submitting your pull request:

• Run cargo fmt (This is checked by travis ci)
• Run cargo clippy and check there are no hard errors (There are a bunch of existing warnings; This is also checked by travis)
• If applicable, add an entry in the changelog.
• If applicable, add documentation to all new items and extend the guide.
• If applicable, add tests for all new or fixed functions
• If you changed any python code, run black .. You can install black with pip install black)

You might want to run tox (pip install tox) locally to check compatibility with all supported python versions. If you're using linux or mac you might find the Makefile helpful for testing.

[davidhewitt]

Thanks, LGTM. I've added a CHANGELOG entry and will merge once CI succeeds.