Update pytest to 3.9.3 (#400)

This PR updates [pytest](https://pypi.org/project/pytest) from **3.9.2** to **3.9.3**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.9.3
   ```
   =========================

Bug Fixes
---------

- `4174 &lt;https://github.com/pytest-dev/pytest/issues/4174&gt;`_: Fix &quot;ValueError: Plugin already registered&quot; with conftest plugins via symlink.


- `4181 &lt;https://github.com/pytest-dev/pytest/issues/4181&gt;`_: Handle race condition between creation and deletion of temporary folders.


- `4221 &lt;https://github.com/pytest-dev/pytest/issues/4221&gt;`_: Fix bug where the warning summary at the end of the test session was not showing the test where the warning was originated.


- `4243 &lt;https://github.com/pytest-dev/pytest/issues/4243&gt;`_: Fix regression when ``stacklevel`` for warnings was passed as positional argument on python2.



Improved Documentation
----------------------

- `3851 &lt;https://github.com/pytest-dev/pytest/issues/3851&gt;`_: Add reference to ``empty_parameter_set_mark`` ini option in documentation of ``pytest.mark.parametrize``



Trivial/Internal Changes
------------------------

- `4028 &lt;https://github.com/pytest-dev/pytest/issues/4028&gt;`_: Revert patching of ``sys.breakpointhook`` since it appears to do nothing.


- `4233 &lt;https://github.com/pytest-dev/pytest/issues/4233&gt;`_: Apply an import sorter (``reorder-python-imports``) to the codebase.


- `4248 &lt;https://github.com/pytest-dev/pytest/issues/4248&gt;`_: Remove use of unnecessary compat shim, six.binary_type
   ```
   
  
</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

@@ -404,9 +404,9 @@ zopfli==0.1.4 \
     --hash=sha256:df7f84a01c0d8c8936e764e63fd0478aa526dc405ee2ad437e633b28cf824141 \
     --hash=sha256:ea204478f5dd4b7c03ec8a25ac5512f5cd85b15e001724289d1676a52c4c1633 \
     --hash=sha256:f5d7840833608f1be3a5f928dbcc0a5ff45ebd8040ea6177864f61a72b773423
-pytest==3.9.2 \
-    --hash=sha256:6f6c1efc8d0ccc21f8f6c34d8330baca883cf109b66b3df954b0a117e5528fb4 \
-    --hash=sha256:212be78a6fa5352c392738a49b18f74ae9aeec1040f47c81cadbfd8d1233c310
+pytest==3.9.3 \
+    --hash=sha256:bf47e8ed20d03764f963f0070ff1c8fda6e2671fc5dd562a4d3b7148ad60f5ca \
+    --hash=sha256:a9e5e8d7ab9d5b0747f37740276eb362e6a76275d76cebbb52c6049d93b475db
 pytest-django==3.4.3 \
     --hash=sha256:49e9ffc856bc6a1bec1c26c5c7b7213dff7cc8bc6b64d624c4d143d04aff0bcf \
     --hash=sha256:b379282feaf89069cb790775ab6bbbd2bd2038a68c7ef9b84a41898e0b551081