Ignore PytestUnknownMark warning when generating docs

[nicoddemus]

Given that our examples use a lot of custom marks, they end up generating a lot of warnings which are only noise in the documentation.

[codecov]

Codecov Report

Merging #5234 into features will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           features    #5234   +/-   ##
=========================================
  Coverage     96.14%   96.14%           
=========================================
  Files           115      115           
  Lines         26125    26125           
  Branches       2577     2577           
=========================================
  Hits          25118    25118           
  Misses          706      706           
  Partials        301      301

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[blueyed]

which are only noise in the documentation.

Where do they show up?
Or would they only without this fix? (when running regendoc without the patch)

[nicoddemus]

They show up in the docs themselves, when we run pytest and capture the output to display on the documentation.

Here's an example git diff when we run tox -e regendoc on features:

diff --git a/doc/en/example/simple.rst b/doc/en/example/simple.rst
index ff422c585..5509612a8 100644
--- a/doc/en/example/simple.rst
+++ b/doc/en/example/simple.rst
@@ -194,10 +194,16 @@ and when running it will see a skipped "slow" test:
     collected 2 items

     test_module.py .s                                                    [100%]
+
+    ============================= warnings summary =============================
+    $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321
+      $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321: 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
     ========================= short test summary info ==========================
     SKIPPED [1] test_module.py:8: need --runslow option to run
-
-    =================== 1 passed, 1 skipped in 0.12 seconds ====================
+    ============= 1 passed, 1 skipped, 1 warnings in 0.12 seconds ==============

 Or run it including the ``slow`` marked test:

@@ -212,7 +218,13 @@ Or run it including the ``slow`` marked test:

     test_module.py ..                                                    [100%]

-    ========================= 2 passed in 0.12 seconds =========================
+    ============================= warnings summary =============================
+    $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321
+      $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321: 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
+    =================== 2 passed, 1 warnings in 0.12 seconds ===================

 Writing well integrated assertion helpers
 --------------------------------------------------
@@ -524,10 +536,16 @@ If we run this:
     E       assert 0

     test_step.py:11: AssertionError
+    ============================= warnings summary =============================
+    $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321
+      $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321: PytestUnknownMarkWarning: Unknown pytest.mark.incremental - 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
     ========================= short test summary info ==========================
     XFAIL test_step.py::TestUserHandling::test_deletion
       reason: previous test failed (test_modification)
-    ============== 1 failed, 2 passed, 1 xfailed in 0.12 seconds ===============
+    ======== 1 failed, 2 passed, 1 xfailed, 1 warnings in 0.12 seconds =========

 We'll see that ``test_deletion`` was not executed because ``test_modification``
 failed.  It is reported as an "expected failure".
@@ -640,7 +658,13 @@ We can run this:
     E       assert 0

     a/test_db2.py:2: AssertionError
-    ========== 3 failed, 2 passed, 1 xfailed, 1 error in 0.12 seconds ==========
+    ============================= warnings summary =============================
+    $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321
+      $PYTHON_PREFIX/lib/python3.6/site-packages/_pytest/mark/structures.py:321: PytestUnknownMarkWarning: Unknown pytest.mark.incremental - 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
+    ==== 3 failed, 2 passed, 1 xfailed, 1 warnings, 1 error in 0.12 seconds ====

 The two test modules in the ``a`` directory see the same ``db`` fixture instance
 while the one test in the sister-directory ``b`` doesn't see it.  We could of course