Update pytest to 4.0.0 (#431)

This PR updates [pytest](https://pypi.org/project/pytest) from **3.10.1** to **4.0.0**.



<details>
  <summary>Changelog</summary>
  
  
   ### 4.0.0
   ```
   =========================

Removals
--------

- `3737 &lt;https://github.com/pytest-dev/pytest/issues/3737&gt;`_: **RemovedInPytest4Warnings are now errors by default.**

  Following our plan to remove deprecated features with as little disruption as
  possible, all warnings of type ``RemovedInPytest4Warnings`` now generate errors
  instead of warning messages.

  **The affected features will be effectively removed in pytest 4.1**, so please consult the
  `Deprecations and Removals &lt;https://docs.pytest.org/en/latest/deprecations.html&gt;`__
  section in the docs for directions on how to update existing code.

  In the pytest ``4.0.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.RemovedInPytest4Warning

  But this will stop working when pytest ``4.1`` is released.

  **If you have concerns** about the removal of a specific feature, please add a
  comment to `4348 &lt;https://github.com/pytest-dev/pytest/issues/4348&gt;`__.


- `4358 &lt;https://github.com/pytest-dev/pytest/issues/4358&gt;`_: Remove the ``::()`` notation to denote a test class instance in node ids.

  Previously, node ids that contain test instances would use ``::()`` to denote the instance like this::

      test_foo.py::Test::()::test_bar

  The extra ``::()`` was puzzling to most users and has been removed, so that the test id becomes now::

      test_foo.py::Test::test_bar

  This change could not accompany a deprecation period as is usual when user-facing functionality changes because
  it was not really possible to detect when the functionality was being used explicitly.

  The extra ``::()`` might have been removed in some places internally already,
  which then led to confusion in places where it was expected, e.g. with
  ``--deselect`` (`4127 &lt;https://github.com/pytest-dev/pytest/issues/4127&gt;`_).

  Test class instances are also not listed with ``--collect-only`` anymore.



Features
--------

- `4270 &lt;https://github.com/pytest-dev/pytest/issues/4270&gt;`_: The ``cache_dir`` option uses ``$TOX_ENV_DIR`` as prefix (if set in the environment).

  This uses a different cache per tox environment by default.



Bug Fixes
---------

- `3554 &lt;https://github.com/pytest-dev/pytest/issues/3554&gt;`_: Fix ``CallInfo.__repr__`` for when the call is not finished yet.
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/pytest
  - Changelog: https://pyup.io/changelogs/pytest/
  - Homepage: https://docs.pytest.org/en/latest/
</details>

requirements.txt

@@ -430,9 +430,9 @@ zopfli==0.1.4 \
     --hash=sha256:f5d7840833608f1be3a5f928dbcc0a5ff45ebd8040ea6177864f61a72b773423
 huey==1.10.4 \
     --hash=sha256:af91e62b48b019080df307d568fbfec452929c6b2c2e7e8eaee622faf7a7cfff
-pytest==3.10.1 \
-    --hash=sha256:3f193df1cfe1d1609d4c583838bea3d532b18d6160fd3f55c9447fdca30848ec \
-    --hash=sha256:e246cf173c01169b9617fc07264b7b1316e78d7a650055235d6d897bc80d9660
+pytest==4.0.0 \
+    --hash=sha256:c055690dfefa744992f563e8c3a654089a6aa5b8092dded9b6fafbd70b2e45a7 \
+    --hash=sha256:488c842647bbeb350029da10325cb40af0a9c7a2fdda45aeb1dda75b60048ffb
 pytest-django==3.4.4 \
     --hash=sha256:deffd9d65827c582bd0a85638a0fe52f0eb65a764872ddcee9ce51cdf6ae9f55 \
     --hash=sha256:fe1f71a0171f6b7edac37654da0904c9bd5ffba5221ab5a76779ab870611f41f