PytestUnknownMarkWarning should reference user code, not pytest

[allanlewis]

When an unknown mark is used, Pytest reports a PytestUnknownMarkWarning. However, this is reported at the level of the warning call, which is obviously in pytest's code, which is unhelpful to users. This warning should be reported at the level of the user's code.

Example:

@pytest.mark.foo
def test_bar():
    pass

$ pytest
...
========================================== warnings summary ===========================================
/home/stb-tester/.local/lib/python2.7/site-packages/_pytest/mark/structures.py:337
  /home/stb-tester/.local/lib/python2.7/site-packages/_pytest/mark/structures.py:337: PytestUnknownMarkWarning: Unknown pytest.mark.slow - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
    PytestUnknownMarkWarning,

-- Docs: https://docs.pytest.org/en/latest/warnings.html
...

• a detailed description of the bug or suggestion
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

[allanlewis]

I'll have a PR for this shortly; I'm only raising an issue so I have a number for the changelog.

[blueyed]

I'll have a PR for this shortly; I'm only raising an issue so I have a number for the changelog.

Cool.
JFI you can also use the PR number then (either guess the next one upfront, or expect having to force-push during the process anyway later).