Test and Dependencies pre-valiation (branch 0.6.x)

[fmigneault]

Motivation

There is no way to know if something breaks or if the generated Docker image still works when doing a PR.
This PR adds a full test suite using Github Actions to validate that deployed images will work.

Changes

• Also apply pyramid>=1.10,<2 (<2 is the important part) that was omitted in PRs fix dependencies issues (5.x) #104 / fix dependencies issues (6.x) #105.
  The pyramid.compat modules is removed since v2 and cannot be found when starting Twitcher.
• Add Github Action workflow that allows testing (local, lint, docker) to ensure features don't break stuff before PR integration.
  (workflow was taken from https://github.com/Ouranosinc/Magpie repo and adjusted/trimmed down for available targets in the current Makefile)
• Add Github Templates to help define issues and PR contents in the future.
• Fix references/format in CHANGES.rst so links are generated both on Github and ReadTheDocs side.

Extra fixes

• Specific to master / 0.6.x branch: resolve incorrect use of decode() with PyJWT>=2 that directly returns strings (issue discovered using above Github test workflow)

See also

For branch 0.5.x: #107

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@29838fa). Click here to learn what that means.
The diff coverage is n/a.

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