Pass DISPLAY variable to tox

tox.ini

@@ -18,11 +18,15 @@ commands = flake8 flatland tests examples benchmarks
 [testenv:docs]
 basepython = python
 whitelist_externals = make
+passenv =
+    DISPLAY
 commands = make docs
 
 [testenv:coverage]
 basepython = python
 whitelist_externals = make
+passenv =
+    DISPLAY
 commands =
     pip install -U pip
     pip install -r requirements_dev.txt
@@ -32,6 +36,8 @@ commands =
 basepython = python
 setenv =
     PYTHONPATH = {toxinidir}
+passenv =
+    DISPLAY
 whitelist_externals = sh
 commands =
     sh -c 'ls benchmarks/*.py  | xargs -n 1 python'
@@ -41,6 +47,8 @@ whitelist_externals = sh
                       pip
 setenv =
     PYTHONPATH = {toxinidir}
+passenv =
+    DISPLAY
 deps =
     -r{toxinidir}/requirements_dev.txt
 ; If you want to make tox run the tests with the same versions, create a