tox: make it possible to set the SPHINXOPTS env var when building the doc with tox

This will allow the CI to run the building job with sphinx options like
`-W` (aka "turn warnings into errors").

docs/Makefile

-SPHINXOPTS =
+SPHINXOPTS ?=
 SPHINXBUILD = python3 -c 'import sphinx, sys; sys.exit(sphinx.main(sys.argv))'
 SOURCEDIR = .
 BUILDDIR = _build

tox.ini

@@ -3,6 +3,7 @@ envlist=flake8
 
 [testenv]
 basepython = python3
+passenv = SPHINXOPTS
 
 [testenv:sphinx]
 deps =