CI: fix CI under Python2.7

test-requirements-27.txt

+attrs==20.3.0
+backports-abc==0.5
+backports.functools-lru-cache==1.6.3
+backports.shutil-get-terminal-size==1.0.0
+bleach==3.3.0
+configparser==4.0.2
+contextlib2==0.6.0.post1
+coverage==5.5
+decorator==4.4.2
+defusedxml==0.7.1
+entrypoints==0.3
+enum34==1.1.10
+functools32==3.2.3.post2
+futures==3.3.0
+importlib-metadata==2.1.1
+ipaddress==1.0.23
+ipykernel==4.10.1
+ipython==5.10.0
+ipython-genutils==0.2.0
+ipywidgets==7.6.3
+Jinja2==2.11.3
+jsonschema==3.2.0
+jupyter==1.0.0
+jupyter-client==5.3.5
+jupyter-console==5.2.0
+jupyter-core==4.6.3
+line-profiler==3.1.0
+MarkupSafe==1.1.1
+mistune==0.8.4
+nbconvert==5.6.1
+nbformat==4.4.0
+notebook==5.7.10
+numpy==1.16.6
+packaging==20.9
+pandocfilters==1.4.3
+pathlib2==2.3.5
+pexpect==4.8.0
+pickleshare==0.7.5
+prometheus-client==0.10.0
+prompt-toolkit==1.0.18
+ptyprocess==0.7.0
+pycodestyle==2.7.0
+Pygments==2.5.2
+pyparsing==2.4.7
+pyrsistent==0.15.7
+python-dateutil==2.8.1
+pyzmq==16.0.4
+qtconsole==4.7.7
+QtPy==1.9.0
+scandir==1.10.0
+Send2Trash==1.5.0
+simplegeneric==0.8.1
+singledispatch==3.6.1
+six==1.15.0
+terminado==0.8.3
+testpath==0.4.4
+tornado==5.1.1
+traitlets==4.3.3
+wcwidth==0.2.5
+webencodings==0.5.1
+widgetsnbextension==3.5.1
+zipp==1.2.0