build fix in the tox

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 = python
+basepython = python3.6
 passenv = DISPLAY
 deps =
     -r{toxinidir}/requirements_dev.txt
@@ -74,7 +74,7 @@ whitelist_externals = sh
 deps =
     -r{toxinidir}/requirements_dev.txt
     -r{toxinidir}/requirements_continuous_integration.txt
-changedir = {toxinidir}    
+changedir = {toxinidir}
 commands =
     python --version
     python {toxinidir}/benchmarks/benchmark_all_examples.py
@@ -97,7 +97,7 @@ conda_channels :
 deps =
     -r{toxinidir}/requirements_dev.txt
     -r{toxinidir}/requirements_continuous_integration.txt
-changedir = {toxinidir}    
+changedir = {toxinidir}
 commands =
     python {toxinidir}/benchmarks/profile_all_examples.py
 
@@ -136,7 +136,7 @@ passenv =
 ; HTTP_PROXY+HTTPS_PROXY required behind corporate proxies
     HTTP_PROXY
     HTTPS_PROXY
-whitelist_externals = sh 
+whitelist_externals = sh
             bash
             pwd
 deps =