feat!: add support for openedx-events

[mariajgrimaldi]

Description
This PR adds support for openedx-events library. Meaning BREAKING CHANGES:

• Refactor each action so it can receive data attr instead of model instances.
• Add tests for Open edX Events receiver hook_handler This function didn't change. We just add new tests for each event it receives.
• Add openedx-events as a dependency.
• Drop support for Python 3.5 (openedx-events requires >= py3.8)

How to test

1. Checkout to edunext-platform branch: li/ednx/JU-6
2. Activate eox-hooks using your tenant settings:
   "USE_EOX_HOOKS": true,
3. For each event, you can use the existent actions. For example:

    "EOX_HOOKS_DEFINITIONS": {
        "post_certificate_creation": {
            "action": "trigger_grades_assignment",
            "fail_silently": true,
            "module": "eox_hooks.actions"
        },
        "post_certificate_creation_webhook": {
            "action": "post_to_webhook_url",
            "config": {
                "extra_fields": {
                    "email_message": "Something extra."
                },
                "fields": {
                    "name": "certificate.user.pii.name",
                    "recipient_email": "certificate.user.pii.email"
                },
                "send_certificate_data": true,
                "url": "https://webhook.site/cba6a96b-0550-4c84-9547-9e210b425b60"
            },
            "fail_silently": true,
            "module": "eox_hooks.actions"
        },
        "post_enrollment": {
            "action": "trigger_enrollments_creation",
            "fail_silently": false,
            "module": "eox_hooks.actions"
        },
        "post_register": {
            "action": "register_user_to_crm",
            "fail_silently": false,
            "module": "eox_taylor.suits.upc.actions"
        }
    },

Notes:
This PR was first opened against edunext/limonero.master: #26