Drop future dependency by using "raise ... from"

[whimboo]

I can see that the mozilla-version package depends on the future package only because of the raise_from usage.

Both instances of the usage actually say:

TODO Use "raise from" statement once Python 2 support is dropped

Given that Python 2 is no longer a thing it would be good to drop the future package from the dependencies because it is quite large in size.

[whimboo]

@JohanLorenzo I would appreciate if this could be fixed. We need to use this package now in mozdownload and I would like to not see a 2MB package needed to be installed. Thanks a lot.

[whimboo]

Thanks for this quick PR! @JohanLorenzo would you have an ETA when the new version could be released?

[JohanLorenzo]

I assume it's going to be soon enough thanks to #104 but I defer to @jcristau.

[jcristau]

This is part of https://pypi.org/project/mozilla-version/2.0.0/