Refactor CI config such that a "run tests" reusable workflow contains one parametrizable job

[sirosen]

The current config has a lot of complexity in ci.yml and that complexity ends up reflecting back into other jobs like cron.yml which try to reuse that job.

A better breakdown:

• a reusable workflow which accepts platform, Python version (should support CPython and pypy), target pip version, ...
  • the scope should be just running the testsuite "correctly" for these inputs
• ci.yml calls this reusable workflow with a matrix of params
• cron.yml calls this workflow with a different (read: smaller) matrix

See also: #2242 (review)

[webknjaz]

A concrete example: https://github.com/tox-dev/workflow/blob/208490c75f7f6b81e2698cc959f24d264c462d57/.github/workflows/reusable-tox.yml

Perhaps, one day we'll just use that.