Add circleci config

[ix5]

Since Travis CI is effectively sunsetting their free plan ¹², there's now a great opportunity to overhaul the whole CI process.

Whereas before travis was only running bare nosetests and flake8, this PR introduces coverage reports and build-checking of the docs as well.

Jobs:

• Run nosetests with coverage on python 3.5 till 3.9
  (There is no werkzeug>=1.0 available on 3.4)
• Run flake8 (currently failing for all builds because of a small syntax error)
• Run sassc to convert .scss files
• Run sphinx to build(-check) docs and upload artifacts
  This enables live viewing of a documentation change!

Smart caching has been utilized so that the whole suite runs in under 2 minutes on the smallest of circle's instances.

For an example of the config in action, have a look at ix5#1

For those of you who have been building on public repositories (on travis-ci.com, with no paid subscription), we will upgrade you to our trial (free) plan with a 10K credit allotment (which allows around 1000 minutes in a Linux environment).

We will be offering an allotment of OSS minutes that will be reviewed and allocated on a case by case basis. Should you want to apply for these credits please open a request with Travis CI support stating that you’d like to be considered for the OSS allotment. Please include:
Your account name and VCS provider (like travis-ci.com/github/[your account name] )
How many credits (build minutes) you’d like to request (should your run out of credits again you can repeat the process to request more or discuss a renewable amount)

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