Split SQL and Python testing in CI

[vincentsarago]

The CI process is a bit complex IMO. It's built on top of a series of shell scripts which call other shell script.

I think it will be good to refactor a bit our CI process to:

• test SQL Only
• test Python Only
• avoid creating migration files every time we want to change the SQL code.

Ideally the migration files should also be created automatically within the CI when pushing a new Tag.

[vincentsarago]

I've started a draft PR over #56 to see how much work it was to just run pgtap test.

Note: to make things as simple as possible (not using docker-compose) I've created postgis docker image but with pgtap included (so we can use them as services in GH actions): https://github.com/developmentseed/postgis-pgtap

[vincentsarago]

The CI is quite complex at the moment and it seems we need the pypgstac to run migration before running pgstac so it makes both python and SQL really tied together, so we can't really split them!