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

docs/conf.py

@@ -61,7 +61,7 @@ html_show_sourcelink = False
 
 def setup(app):
     # overrides for wide tables in RTD theme
-    app.add_stylesheet('theme_overrides.css')   # path relative to _static
+    app.add_css_file('theme_overrides.css')   # path relative to _static
 
 
 # -- Options for LaTeX output -------------------------------------------------

docs/themes.rst

@@ -242,7 +242,7 @@ as the name of the metadata field, except in all-lowercase characters.
 For example, you could add a field called `FacebookImage` to your article
 metadata, as shown below:
 
-.. code-block:: markdown
+.. code-block:: md
 
     Title: I love Python more than music
     Date: 2013-11-06 10:06