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

requirements/docs.pip

@@ -1,3 +1,3 @@
-sphinx==1.4.9
+sphinx
 sphinx_rtd_theme
 livereload

requirements/test.pip

@@ -1,6 +1,5 @@
 # Tests
 Pygments==2.6.1
-mock
 pytest==5.3.5
 pytest-cov
 pytest-xdist