Problem with partial and pickle

I'm not using astroid directly, but discovered this issue [downstream](https://github.com/readthedocs/sphinx-autoapi/issues/264) using sphinx-autoapi.
It seems I've run into an edge case with `partial` and the `pickle` module. I've added a new test case to `astroid` to showcase my problem: https://github.com/PyCQA/astroid/compare/master...renefritze:conditional_test_pickle

With the last commit on that branch the test suite succeeds: https://travis-ci.com/github/renefritze/astroid/jobs/487475875
Dropping the last commit, the suite fails: https://travis-ci.com/github/renefritze/astroid/jobs/487452325
The difference being https://github.com/PyCQA/astroid/commit/307f3acbfaa8bac3e7e4f1f0d0f38a965e54c53d

Compare the linked conditional_test_pickle branch with master, focusing on the added astroid test and commit 307f3acbfaa8bac3e7e4f1f0d0f38a965e54c53d. Run the relevant test suite with and without that commit to reproduce the partial and pickle failure. Done means the reported test passes without the workaround.

Written by the indexing model from the issue text.