Fix python version cfg

[pejter]

No description provided.

[pejter]

Fixed the clippy warnings.

[aviramha]

We need to follow those instructions to have the supplemented PyO3 cfgs.

[pejter]

So why does it work without it at the moment? If the dependency isn't present and the flags are missing then something should fail, right? I tested the 3.6 version locally and it does indeed fail at import, so perhaps there should be a very basic python-level test suite in place to verify the API?

[aviramha]

@pejter It is odd. I'm working on adding a test to verify declaration works as desired.
#27

[aviramha]

It seems that it does fail. Your changes should fix it.
I took the liberty to do the build.rs changes, hope that's okay :)

[aviramha]

Thanks!