Regression in 2.7.1 when validating notebooks

[goerz]

I am running pytest for the krotov package with both pytest-cov and the nbval plugin to validate jupyter notebook in the documentation. Since pytest-cov was updated to version 2.7.1, there is extra output related to repr-strings of internal coverage objects appearing in the output of some notebook cells. See https://travis-ci.org/goerz/krotov/jobs/528562228 for the failure, and compare this with the working run at https://travis-ci.org/qucontrol/krotov/jobs/527616527. The first failure is for Cell 14 of https://krotov.readthedocs.io/en/latest/notebooks/05_example_transmon_xgate.html. I'm able to reproduce the problem locally (both on macOS and Linux), not just on Travis, and I can also verify that the problem disappears if I pin pytest-cov to version 2.6.1 in krotov's setup.py.

[goerz]

See also the working test runs on Travis after pinning pytest-cov to 2.6.1: https://travis-ci.org/qucontrol/krotov/jobs/528675984 and https://travis-ci.org/goerz/krotov/jobs/528676409 (the latter one fixing the previous broken run https://travis-ci.org/goerz/krotov/jobs/528562228 on the same repo)