Asphalt-mailer

- **BACKWARD INCOMPATIBLE** Bumped minimum Asphalt version to 4.8
- **BACKWARD INCOMPATIBLE** Bumped minimum aiosmtplib version to 2.0
- **BACKWARD INCOMPATIBLE** Refactored component to only provide a single mailer
(you will have to add two components to get two serializers)
- **BACKWARD INCOMPATIBLE** Dropped the context attribute (use dependency injection
instead)
- Dropped explicit run-time type checking

- Fixed SMTP connection staying open when a ``DeliveryError`` is raised

- Retry of the previous release, with all reported code style errors fixed

- Fixed deprecation warning about importing ABCs directly from ``collections``

- Added compatibility with Asphalt 4.0

- **BACKWARD INCOMPATIBLE** Migrated to Asphalt 3.0
- **BACKWARD INCOMPATIBLE** Replaced home grown SMTP implementation with aiosmtplib
- **BACKWARD INCOMPATIBLE** Implicit TLS support in SMTPMailer was replaced with STARTTLS support
- **BACKWARD INCOMPATIBLE** The ``ssl`` option in SMTPMailer was replaced with the ``tls`` and
``tls_context`` options
- **BACKWARD INCOMPATIBLE** Default port selection in SMTPMailer was changed; see the class
docstring for details
- **BACKWARD INCOMPATIBLE** Renamed the ``asphalt.mailer.util`` module to ``asphalt.mailer.utils``