- **BACKWARDS INCOMPATIBLE** The concrete client implementations were refactored:
- `AsyncSMTPClient` and `SyncSMTPClient` were refactored into "session factories", and thus are no longer used as context managers
- The `send_message()` method is now reentrant, as it now creates (and closes) an ad-hoc session with the SMTP server
- The `connect()` method now returns a context manager that yields an SMTP session
- **BACKWARDS INCOMPATIBLE** The `OAuth2Authenticator` class was refactored:
- The return type of `get_token()` was changed to a (decoded) JSON web token – a dict containing the `access_token` and `expires_in` fields
- The result of `get_token()` method is now automatically cached until the token's expiration time nears (configurable via the `grace_period` parameter in `OAuth2Authenticator`)
- Added the `clear_cached_token()` method
- Dropped support for Python 3.7
- Upgraded minimum AnyIO version to 4.2+
- The `Bcc` and `Resent-Bcc` are now properly added to the recipients list by the concrete client implementation
- The `Bcc` and `Resent-Bcc` headers are now automatically left out of the data in `SMTPClientProtocol.data()` to simplify client implementations