Celery-yaml

* Update README
* Update the CI

* Remove setuptools' `pkg_resources` to resolve entrypoints for pyramid.

* Now configuration can contains environment variable that

will be replaced before configurating the app.
* Only ${VARIABLE} will be replaced if present in the os.environ
* A default value can be specified using ${VARIABLE-default-value}

This feature is great at injecting secrets in the configuration
and keep code simple.

* A `--yaml-key` has been added to configure the celery configuration dict.
by default its still `celery`.

* Drop support of python < 3.9
* Drop support of Celery 4

* Remove the bootstep for celery 5, it just not works.

* Tests with celery 5, keep celery 4