Move CI from Travis to GitHub

[janbrasna]

Removes Travis config and creates a GHA matrix of a handful CPython versions to run cipherscan against example.org for both master pushes as well as PRs open (+manual runs from UI).

Resolves #187

(Also swaps contributor credits for dynamic avatars based on contrib graph, keeping only authors/maintainers for easier maintenance.)

[janbrasna]

(Example run for d84b4bb↗️…)

[tomato42]

looks good, but I don't seem to have permission to enable Actions, @jvehent could you?

[janbrasna]

@tomato42 Confirmed with GHE they should be already enabled and would trigger right after merging. (Or do you seem to be getting any errors regarding the merge, file/folder warnings et al.?)

[tomato42]

I don't see a button to trigger a CI run. (something I do see in repos I have the admin privileges for)

[janbrasna]

@tomato42 Oh I believe this has to land first. Only once pushed by someone with the perms written to master, then the CI gets picked up for subsequent runs, new PRs off of master and subsequent tags. (I. e. you won't get the workflows trigger on anything older than when it landed in the default branch, incl. the PR trying to actually land it; was wrestling with that too recently wrt tags/releases — the action is triggered by the sha ref, and if that ref doesn't already include the workflow in the tree it won't be triggered even when already in the base;D)

I will then open a subsequent PR hopefully updating the specs version given nothing in the data logic breaks, and that will trigger another approval, "workflows awaiting run approvals from first-time contributors" of sorts (or a switch to enable it for all forks), so you'll have that one pending button too then, to see if you have the perms to also approve runs for outsiders.

[tomato42]

It may be so, it all depends how Actions are set up. I can't check it, @jvehent should be able to.