Enable sphinx-build --color by default

And add environment variable SPHINXOPTCOLOR to disable it on Jenkins

docs/Makefile

 SPHINXOPTS ?=
+SPHINXOPTCOLOR ?= --color
 SPHINXBUILD = sphinx-build
 SOURCEDIR = .
 BUILDDIR = _build
@@ -63,10 +64,10 @@ distclean: clean
 help: sphinx/help
 
 sphinx/clean:
-	$(SPHINXBUILD) -M clean "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS)
+	$(SPHINXBUILD) -M clean "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(SPHINXOPTCOLOR)
 
 sphinx/%:
-	$(SPHINXBUILD) -b $* "$(SOURCEDIR)" "$(BUILDDIR)/$*" $(SPHINXOPTS)
+	$(SPHINXBUILD) -b $* "$(SOURCEDIR)" "$(BUILDDIR)/$*" $(SPHINXOPTS) $(SPHINXOPTCOLOR)
 
 install: html
 	test -d $(HTMLDIR)

tox.ini

@@ -3,7 +3,7 @@ envlist=flake8,black
 
 [testenv]
 basepython = python3
-passenv = SPHINXOPTS JAVA_HOME
+passenv = SPHINXOPTS SPHINXOPTCOLOR JAVA_HOME
 
 [testenv:sphinx]
 extras =