[#2367] Fix string listing in errors.

Problem
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When string differs due to non printable characters it is hard to see the different from the reported error.

Changes
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I have forced the usage of repr() instead of str() in AssertionError messages

How to test
========

reviewers: @alibotean 

There is no automated test, but for example you can try to assert `bla\n` and 'bla\r\n` ... in the old version you will see that they differ... but the printed output look the same.

I have released the version and already used in on server in a branch and it looks ok.

Thanks!