Add license_files to setup.py

[adriangonz]

What this PR does / why we need it:
Add license_files key to setup.py so that the dist-info folder generated by setuptools includes Seldon's license file.

Which issue(s) this PR fixes:

Fixes #2124

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[adriangonz]

/cc @cliveseldon @RafalSkolasinski

[seldondev]

Tue Jul 28 14:06:15 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2210/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2210 --build=1

[seldondev]

Tue Jul 28 14:06:51 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2210/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2210 --build=2

[axsaucedo]

/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

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