Do full Sphinx build every time to not hide errors (#322)

Author: Oberon00

tox.ini

@@ -160,7 +160,7 @@ deps =
 changedir = docs
 
 commands =
-  sphinx-build -W --keep-going -b html -T . _build/html
+  sphinx-build -E -a -W --keep-going -b html -T . _build/html
 
 [testenv:py37-tracecontext]
 basepython: python3.7