Requirement is pinned for saltbox 0.1.3: certifi==2019.11.28

[MartinHjelmare]

• This library supports an integration in Home Assistant.
• The upcoming pip resolver will be more strict in resolving package version conflicts:
  http://pyfound.blogspot.com/2020/03/new-pip-resolver-to-roll-out-this-year.html#what-will-change
• To avoid future issues it would be good if this library would change its pinning to allow any higher version and just set a minimum version.

Thanks for your support of Home Assistant!