[scikit-learn] make all new parameters keyword-only?

[Andreas Mueller]

Hi all.
Since we'll be dropping Python2.7 soon, we can now use keyword-only 
arguments.
I would argue that whenever we add any argument anywhere, we should make 
it keyword-only from now on,
with the exception of X and y (probably).
What do others think?
Are there other features in Python3 that we should consider adopting for 
0.21?

The reason for making arguments keyword-only is that
a) users are force to write more readable code
b) deprecations and api changes have less side-effects

Cheers,
Andy