Avoid storing bound methods as variables to prevent pickling problems

[ghost]

Fixes the problem with tests on Python 2.7 for #2201.

[ghost]

@junpenglao Yes, it works on top of #2201.

[junpenglao]

thanks @a-rodin, i accidentally deleted my question. So it need to wait for #2201 to merge first right?

[ghost]

Not necessary, the target branch of this PR is latex_dists, so the PR can be just merged into that branch and the commit would appear in #2201 after the merge.

[twiecki]

@fonnesbeck This is a fix for your open PR to make it python 2 compatible.

[fonnesbeck]

Thanks!