Reinstate python2 only constraint

[petemounce]

See gitter chat around https://gitter.im/chocolatey/choco?at=58361d42b07a1fd34833fe36. The rc failed automatic validation because of it, we think.

[bazel-io]

Can one of the admins verify this patch?

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[petemounce]

@meteorcloudy #2122 ran into a bug with either chocolatey or the python packages there, so for now, reinstating the upper constraint.

[meteorcloudy]

I see, LGTM.