Errors with flake8 running tox

## Location within the Course

* Section:4
* Lesson:2
* Language:en

## Description

When running tox, I now get errors in the flake8 configuration. They are SSL errors. Tried several things to no avail. I will tell my students to remove flake8 from the `envlist` in tox.ini.
Happens with local dev environment as 3.6 and 3.7. Pip is 18.1 already.

Here is the start of the error message:
flake8 inst-nodeps: /Users/diane/projects/squashem_pkg/squashem/.tox/.tmp/package/1/squashem-0.1.0.zip
ERROR: invocation failed (exit code 1), logfile: /Users/diane/projects/squashem_pkg/squashem/.tox/flake8/log/flake8-4.log
ERROR: actionid: flake8
msg: installpkg
cmdargs: '/Users/diane/projects/squashem_pkg/squashem/.tox/flake8/bin/python -m pip install --no-deps -U /Users/diane/projects/squashem_pkg/squashem/.tox/.tmp/package/1/squashem-0.1.0.zip'

Processing ./.tox/.tmp/package/1/squashem-0.1.0.zip
Complete output from command python setup.py egg_info:
Download error on https://pypi.org/simple/pytest-runner/: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1045) -- Some packages may not be found!
Couldn't find index page for 'pytest-runner' (maybe misspelled?)
Download error on https://pypi.org/simple/: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1045) -- Some packages may not be found!
No local packages or working download links found for pytest-runner

Inizia da tox.ini e dall’ambiente flake8 descritto nell’issue, quindi riproduci l’esecuzione di tox su Python 3.6 o 3.7. Traccia il fallimento dell’installazione di pytest-runner e l’errore del certificato SSL; il lavoro è completato quando la configurazione documentata del corso esegue tox correttamente senza richiedere agli studenti di rimuovere flake8.

Scritto dal modello di indicizzazione a partire dal testo della issue.