Cleanup test environment and requirements

- pytest: verbose isn't needed but show skipped tests
- mock is not needed anymore. it is part of stdlib
- Unpin Sphinx and fix warning about lexer
- Mirror tox command for doc build in tasks.py

tox.ini

@@ -14,7 +14,7 @@ deps =
 
 commands =
     {envpython} --version
-    pytest -sv --cov=pelican pelican
+    pytest -s --cov=pelican pelican
 
 [testenv:docs]
 basepython = python3.6
@@ -28,7 +28,7 @@ commands =
 filterwarnings =
     default::DeprecationWarning
     error:.*:Warning:pelican
-addopts = -n 2
+addopts = -n 2 -r a
 
 [flake8]
 application-import-names = pelican