Smtpdfix

Updates to use the latest release of `aiosmtpd` and to automatically load as a plugin with `pytest`.

- Adds an `SMTPDFix` class for use in `with` blocks.
- The fixture now automatically loads as a plugin with pytest.
- Aiosmtpd version 1.2.4 or greater now required.
- Command responses now standard with aiosmtpd with the exception of AUTH mechanisms.
- Prefers StartTLS to TLS/SSL to avoid a case where if both where specified the SMTP server would report a fault.
- Minor performance enhancements from removing the lazy object proxy and setting key size to 2048 bits instead of 4096.
- Allows for late changes to StartTLS environment variables.

**Version yanked from pypi because it allowed for insecure versions of cryptography to be used.**

This release has been motivated by recent breaking changes to aiosmtpd version 1.2.3 and above.

- Certificates are now dynamically generated per session rather than stored in the package.
- Enforces aiosmtpd to version 1.2.2 or below.

**Version yanked from pypi because it allowed for insecure versions of cryptography to be used.**

Adds support for mocking environment variables in some cases and corrects errors

+ Corrects an error whereby running without a .env file setting true/false environment variables produced an error.
+ Allows for some, but not all, environment variables to be set after test setup.

**Version yanked from pypi because it allowed for insecure versions of cryptography to be used.**

Adds support for python 3.9 and limits `aiosmtpd` to prevent future changes creating incompatibilities.

- Add support for `async` tests and `coverage`.
- Fixes support for .env files and makes treatment of environment variables more consistent.

**Version yanked from pypi because it allowed for insecure versions of cryptography to be used.**

Corrects errors to Version 0.2.3 which prevented it from running under `smtpdfix`.

- Corrects path in `setup.py` so that certificates are included.
- Updates the certificates so that they're good until at least August 2, 2030

**Version yanked from pypi because it allowed for insecure versions of cryptography to be used.**

Final release as bebleo_smtpd_fixture before becoming smtpdfix.

- Adds deprecation warning to the fixture letting developers know that if by chance they're using bebelo_smtpd_fixture they should move to smtpdfix