Move ml deps into extras

[kalekseev]

A lot of people would like to use talon without ml and it's possible to move the ml deps into extras and install it with pip install talon[ml] while pip install talon will install light version without ml.
I've implemented such setup and added tox which tests both versions.
This is a breaking change so I'm interested in would you accept such change if I finish all required things for the pr?

[mailgun-ci]

Can one of the admins verify this patch?

[mnba]

And a lot of people would like to use (and now really use) talon with ML
and it would be better someone repair talon.signature cross-circular dependency
while not removing/extracting ML part only because of corrupt part

I proposed solution to that in some comment here, while didn't provide PR

[kalekseev]

@MinasAbrahamyan The cross-circular dependency is not the point of this pr. In fact I didn't know that the problem exists. I use talon in some my projects and I don't want to donwload and compile numpy, scipy, scikit and os packages for the small amount of cases when the ml is helpful. And you can see several issues in the project there people don't want to install ml part. There is even the --no-ml flag in the setup.py which do this but it's not very convenient to use it with pip.
I don't expect this PR will be accepted in the current state. This is more a proof of concept that ml can be moved into the extras and my proposal to work on this if authors agree to make this change.

[obukhov-sergey]

@kalekseev thank you for the PR and my apologies for delay with response. It should already be possible to install / use the lib without ML support if you install with --no-ml option. Here's the commit.

[kalekseev]

@obukhov-sergey I know about this option but because this is not a standard way to install optional deps in pip/python it leads to problem like this pypa/pip#3845
Even the author of the --no-ml patch doesn't use this option anymore zulip/zulip@624f79a#diff-dc7e3ad1b4e905601b3a4311ceb1f5ec