Introduce version 3.6 - rebased

[chrisburr]

This PR is simply #208 rebased to fix the conflicts with an additional commit to address the comments of @torsava.

If you would rather close this and rebase/merge #208 instead I am be happy with that, I just did this as I needed to build an image using Python 3.6.

[rhscl-bot]

Can one of the admins verify this patch?

[omron93]

[test]

[centos-ci]

Can one of the admins verify this patch?

[torsava]

Looks good to me, thanks @chrisburr! Unless there are any objections, I'll merge this.

[pkubatrh]

@chrisburr Please add the 3.6 version string into the Makefile over at:
https://github.com/chrisburr/s2i-python-container/blob/introduce-36-rebase/Makefile#L3

If this is not set the CI tests are not run against the new version (and having .exclude-rhel7 is pointless in that case).

[pkubatrh]

Also needs a rebase before being able to be merged

[chrisburr]

@pkubatrh Done

[pkubatrh]

[test]

[pkubatrh]

Seems like the 3.6 scl rpms are already in centos-sclo-rh-testing. @chrisburr please also remove this line to make the CentOS tests pass:
https://github.com/chrisburr/s2i-python-container/blob/introduce-36-rebase/3.6/Dockerfile#L37

[pkubatrh]

[test]

[pkubatrh]

Tests are green now, thanks! LGTM.