Prepare Halloween release 0.1.8 🎃

[gmaze]

Prepare release

1. Make sure that all CI tests are passed with free environments
2. Update ./requirements.txt and ./docs/requirements.txt with CI free environments dependencies versions
3. Update ./ci/requirements/py*-dev.yml with last free environments dependencies versions
4. Make sure that all CI tests are passed with dev environments
5. Increase release version in ./setup.py file
6. Update date and release version in ./docs/whats-new.rst
7. Merge this PR to master

Publish release

• Make sure that all CI tests are passed

• On the master branch, commit the release in git:
  git commit -a -m 'Release v0.X.Y'

• Tag the release:
  git tag -a v0.X.Y -m 'v0.X.Y'

• Push it online:
  git push origin v0.X.Y

• Issue the release on GitHub. Click on "Draft a new release" at
  https://github.com/euroargodev/argopy/releases. Type in the version number, but
  don't bother to describe it -- we maintain that on the docs instead.

  This will trigger the publish Github action that will push the release on Pypi.