[gdal-dev] Migration of Travis-CI jobs to GitHub actions & Python >= 3.6

[Even Rouault]

Hi,

I've worked this week on migrating most of our Travis-CI jobs to GitHub 
actions.

Travis-CI future doesn't seem very promising IMHO, due to the way they behave 
commercially. They have recently changed their conditions, mostly for people 
using their free offer, but that affects even GDAL despite OSGeo having a paid 
subscription to them. After next June, we won't be able to get the 
same quality of service we have currently without paying at least 3 times more 
(and maybe much more), or we'd have to choose an offer with less concurrent 
workers (5 instead of 10 currently).

So I've moved Travis CI configs to github actions (dropping a few no longer 
relevant ones). The builds are now done in standard Docker images (Ubuntu 
ones) instead of the one provided by the CI provider, so it should make it 
easier to move to another CI if/when GHA will try to monetize us, and more 
importantly it's much easier to replicate a CI job locally for debugging.

For example to run the ubuntu:18.04 job, from the top of a GDAL worktree (use 
a dedicated git worktree since that will modify your tree in the host 
environment)

docker run --rm -it --privileged=true -e WORK_DIR="$PWD" -v $PWD:$PWD \
	-v /var/run/docker.sock:/var/run/docker.sock \
	ubuntu:18.04 /bin/bash

and inside the container:
$WORK_DIR/github/workflows/ubuntu_18.04/start.sh

(that one needs priviledged docker, since it run docker containers inside 
docker)

The only remaining Travis-CI job is the s390x one, that has no equivalent in 
GHA. OSGeo might perhaps just subscribe to the base 1-concurrent job tier to 
keep that one.

When moving & updating the jobs, I've also updated to Python >= 3.6 for, 
hopefully, all targets that didn't meet this yet. So we should be good to go 
for RFC 77

Even

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