Try passenv DISPLAY again

tox.ini

@@ -13,6 +13,7 @@ ignore = E121 E126 E123 E128 E133 E226 E241 E242 E704 W291 W293 W391 W503 W504 W
 [testenv:flake8]
 basepython = python
 deps = flake8
+passenv = DISPLAY
 commands = flake8 flatland tests examples benchmarks
 
 [testenv:docs]
@@ -25,8 +26,7 @@ commands = make docs
 [testenv:coverage]
 basepython = python
 whitelist_externals = make
-passenv =
-    DISPLAY
+passenv = DISPLAY
 commands =
     pip install -U pip
     pip install -r requirements_dev.txt
@@ -36,8 +36,7 @@ commands =
 basepython = python
 setenv =
     PYTHONPATH = {toxinidir}
-passenv =
-    DISPLAY
+passenv = DISPLAY
 whitelist_externals = sh
 commands =
     sh -c 'ls benchmarks/*.py  | xargs -n 1 python'
@@ -47,8 +46,7 @@ whitelist_externals = sh
                       pip
 setenv =
     PYTHONPATH = {toxinidir}
-passenv =
-    DISPLAY
+passenv = DISPLAY
 deps =
     -r{toxinidir}/requirements_dev.txt
 ; If you want to make tox run the tests with the same versions, create a