fix issue #2920

[rpuntaie]

See issue #2920 for details.

[nicoddemus]

@rpuntaie thanks for tackling this!

We need to add a new test for it though. I suggest writing a test in test_config.py or acceptance_test.py; take a look at other tests using the testdir fixture, it should be straightforward to prepare an integration test which triggers the bug.

[coveralls]

Coverage remained the same at 92.688% when pulling 4a840a7 on rpuntaie:master into 4d2f05e on pytest-dev:master.

[rpuntaie]

test_addopts_before_initini fails, but I think that has no relation to my changes. The changed test test_plugin_preparse_prevents_setuptools_loading fails without my fix and it succeeds with my fix.

[coveralls]

Coverage remained the same at 92.688% when pulling 4a840a7 on rpuntaie:master into 4d2f05e on pytest-dev:master.

[coveralls]

Coverage remained the same at 92.688% when pulling 3dbac17 on rpuntaie:master into 4d2f05e on pytest-dev:master.

[coveralls]

Coverage remained the same at 92.688% when pulling 833f33f on rpuntaie:master into 4d2f05e on pytest-dev:master.

[nicoddemus]

Thanks @rpuntaie! I verified locally that your test reproduces the problem and also improved it a bit to ensure all the mechanisms that we are mocking in that test work as expected when we don't use the -p no: flag.

[coveralls]

Coverage remained the same at 92.688% when pulling 5f1a733 on rpuntaie:master into 4d2f05e on pytest-dev:master.

[The-Compiler]

Thanks @rpuntaie and @nicoddemus!