Simplify CI and use cache

[janiversen]

Adding a "modernised" version of ci.yml, it is basically a new setup:

• simpler with 1 job and a matrix
• caching dependencies (reducing runtime by around 50%)
• no upload of coverage
• no special architecture testing

The new CI can be manually activated on a branch:

Also with the new CI, future changes will be done in a branch and not merged until ready and carefully tested.

There are no longer support for tox, since the linter tools and purest can be run directly. A new script ./check_ci.sh are added to allow local control of a commit.

[janiversen]

@dhoomakethu this PR might have an impact on the way you work (no tox), but should make it a lot easier for you to test on a branch e.g. using workspaces.