automatic test build fix

tox.ini

@@ -12,7 +12,7 @@ max-line-length = 120
 ignore = E121 E126 E123 E128 E133 E226 E241 E242 E704 W291 W293 W391 W503 W504 W505
 
 [testenv:flake8]
-basepython = python3.6
+basepython = python3.7
 passenv = DISPLAY
 deps =
     -r{toxinidir}/requirements_dev.txt
@@ -21,7 +21,7 @@ commands =
     flake8 flatland tests examples benchmarks
 
 [testenv:docs]
-basepython = python3.6
+basepython = python3.7
 whitelist_externals = make
 passenv =
     DISPLAY
@@ -41,7 +41,7 @@ commands =
     make docs
 
 [testenv:coverage]
-basepython = python3.6
+basepython = python3.7
 whitelist_externals = make
 passenv =
     DISPLAY
@@ -61,7 +61,7 @@ commands =
     python make_coverage.py
 
 [testenv:benchmarks]
-basepython = python3.6
+basepython = python3.7
 setenv =
     PYTHONPATH = {toxinidir}
 passenv =
@@ -80,7 +80,7 @@ commands =
     python {toxinidir}/benchmarks/benchmark_all_examples.py
 
 [testenv:profiling]
-basepython = python3.6
+basepython = python3.7
 setenv =
     PYTHONPATH = {toxinidir}
 passenv =
@@ -103,7 +103,7 @@ commands =
 
 [testenv:examples]
 ; TODO should examples be run with py36 and py37??
-basepython = python3.6
+basepython = python3.7
 setenv =
     PYTHONPATH = {toxinidir}
 passenv =
@@ -126,7 +126,7 @@ commands =
 
 [testenv:notebooks]
 ; TODO should examples be run with py36 and py37??
-basepython = python3.6
+basepython = python3.7
 setenv =
     PYTHONPATH = {envdir}
     ;{toxinidir}
@@ -159,7 +159,7 @@ commands =
     python {toxinidir}/notebooks/run_all_notebooks.py
 
 [testenv:start_jupyter]
-basepython = python3.6
+basepython = python3.7
 setenv =
     PYTHONPATH = {toxinidir}
 passenv =