Display test name in warning message when Monkeypatch.setenv/delenv use bytes keys

[nicoddemus]

From #4056 (comment) by @gaborbernat:

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Hey guys, would it be possible to also report at least the test that catches this? In tox now I get:

018-10-05T09:23:49.0704702Z =============================== warnings summary ===============================
2018-10-05T09:23:49.0705517Z /home/vsts/work/1/s/.tox/py27/lib/python2.7/site-packages/_pytest/monkeypatch.py:232: PytestWarning: Environment variable value local('/tmp/pytest-of-vsts/pytest-0/popen-gw0/test_substitution_jenkins_cont0') should be str, converted to str implicitly
2018-10-05T09:23:49.0705945Z   value
2018-10-05T09:23:49.0706652Z /home/vsts/work/1/s/.tox/py27/lib/python2.7/site-packages/_pytest/monkeypatch.py:232: PytestWarning: Environment variable value 1 should be str, converted to str implicitly

I have 450+ tests though leaving me puzzled where to fix it...

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Seems easy, Monkeypatch might receive an optional nodeid object that is used in the message.

[nicoddemus]

Note: We should also use the correct stack level

[nicoddemus]

Fixed by 5404246