Define pytest marks (#161)

* Define pytest marks

This is to avoid a WARNING,

tests/test_versions.py:7

  /home/travis/build/readthedocs/readthedocs-docker-images/tests/test_versions.py:7: PytestUnknownMarkWarning: Unknown pytest.mark.image_master - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/mark.html

    @pytest.mark.image_master

* Update pytest.ini

Co-authored-by: Santos Gallegos <[email protected]>

Co-authored-by: Santos Gallegos <[email protected]>

pytest.ini

@@ -0,0 +1,7 @@
+[pytest]
+addopts = --strict-markers
+markers =
+    image_master
+    image_70
+    image_60
+    image_50