use single source for package version & bump it

[hiaselhans]

This pr bumps the version to 1.3.16 for a new release (#52)

additionally it puts the version number in a separate python file and setup reads from there (single source of truth)

Thanks for the great library! let's keep it updated...

[martinleblanc]

Thanks for the PR. We'll test it and merge if all good.

[hiaselhans]

@martinleblanc did you manage to test it?

tests are failing in the current master and require #54 to work again. Thanks!

[ml31415]

@martinleblanc Have you thought about adding another maintainer, to keep this project up and running and give it the necessary love and care, that this package deserves? Maybe @hiaselhans would be interested.

[hiaselhans]

sorry @ml31415 i am out. i made this library and use it instead: https://github.com/airgproducts/eurovat