Tox passing

requirements.txt

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+backcall==0.1.0
+certifi==2018.11.29
+chardet==3.0.4
+decorator==4.3.2
+filelock==3.0.10
+idna==2.8
+ipython-genutils==0.2.0
+networkx==2.2
+numpy==1.16.2
+parso==0.3.4
+pexpect==4.6.0
+pickleshare==0.7.5
+pluggy==0.9.0
+prompt-toolkit==2.0.9
+ptyprocess==0.6.0
+py==1.8.0
+Pygments==2.3.1
+requests==2.21.0
+six==1.12.0
+toml==0.10.0
+tox==3.7.0
+traitlets==4.3.2
+urllib3==1.24.1
+virtualenv==16.4.3
+wcwidth==0.1.7

tox.ini

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+[tox]
+envlist = py27, py36
+
+[testenv]
+deps=
+    -r requirements.txt
+    pytest
+commands=pytest {posargs}