Turn PytestDeprecationWarning into error (#5410)

Turn PytestDeprecationWarning into error

changelog/5402.removal.rst

@@ -0,0 +1,23 @@
+**PytestDeprecationWarning are now errors by default.**
+
+Following our plan to remove deprecated features with as little disruption as
+possible, all warnings of type ``PytestDeprecationWarning`` now generate errors
+instead of warning messages.
+
+**The affected features will be effectively removed in pytest 5.1**, so please consult the
+`Deprecations and Removals <https://docs.pytest.org/en/latest/deprecations.html>`__
+section in the docs for directions on how to update existing code.
+
+In the pytest ``5.0.X`` series, it is possible to change the errors back into warnings as a stop
+gap measure by adding this to your ``pytest.ini`` file:
+
+.. code-block:: ini
+
+    [pytest]
+    filterwarnings =
+        ignore::pytest.PytestDeprecationWarning
+
+But this will stop working when pytest ``5.1`` is released.
+
+**If you have concerns** about the removal of a specific feature, please add a
+comment to `#5402 <https://github.com/pytest-dev/pytest/issues/5402>`__.