Update PyO3 to 0.7 and release 0.6

[kngwyu]

This release drops Python2 support

[rth]

Thanks for making this release @kngwyu ! I quickly went through the pyo3 changelog and coudn't see any new features that would require additional code changes here (in my limited understanding of pyo3).

[kngwyu]

Thanks for the review 😃