Update pytest to 3.9.3 (#1273)

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

@@ -207,9 +207,9 @@ flake8==3.5.0 \
 mock==2.0.0 \
     --hash=sha256:5ce3c71c5545b472da17b72268978914d0252980348636840bd34a00b5cc96c1 \
     --hash=sha256:b158b6df76edd239b8208d481dc46b6afd45a846b7812ff0ce58971cf5bc8bba
-pytest==3.9.2 \
-    --hash=sha256:6f6c1efc8d0ccc21f8f6c34d8330baca883cf109b66b3df954b0a117e5528fb4 \
-    --hash=sha256:212be78a6fa5352c392738a49b18f74ae9aeec1040f47c81cadbfd8d1233c310
+pytest==3.9.3 \
+    --hash=sha256:bf47e8ed20d03764f963f0070ff1c8fda6e2671fc5dd562a4d3b7148ad60f5ca \
+    --hash=sha256:a9e5e8d7ab9d5b0747f37740276eb362e6a76275d76cebbb52c6049d93b475db
 coverage==4.5.1 \
     --hash=sha256:7608a3dd5d73cb06c531b8925e0ef8d3de31fed2544a7de6c63960a1e73ea4bc \
     --hash=sha256:3a2184c6d797a125dca8367878d3b9a178b6fdd05fdc2d35d758c3006a1cd694 \