Switch Tox test runner from Nose to Pytest

tox.ini

@@ -11,14 +11,14 @@ passenv = *
 usedevelop=True
 deps =
     -rrequirements/test.pip
-    nose
-    nose-cov
+    pytest
+    pytest-cov
     coveralls
     pygments==2.1.3
 
 commands =
     {envpython} --version
-    nosetests -sv --with-coverage --cover-package=pelican pelican
+    pytest -sv --cov=pelican pelican
     - coveralls
 
 [testenv:docs]