Update pyo3 to 0.8 and bump version to 0.7

[kngwyu]

TODO

• Modify Cargo.toml when 0.8 is released
• Check if the module name is appropriate again

[rth]

Before the next rust-numpy release, maybe it could make sense to also wait for the release of ndarray 0.13 expected early September rust-ndarray/ndarray#670)?

[kngwyu]

Great, thank you for letting me know that.

[kngwyu]

@rth
Since ndarray 0.13 is not yet released, I decided to release rust-numpy 0.7 before the release.
I confirmed that the latest master of ndarray works with rust-numpy and changed the Cargo.toml to ndarray >= 0.12.

[rth]

Sure, that also works. Thanks @kngwyu !