C# SMTPClient uses lowercase login AUTH method

Hello,

I am currently implementing a smtp handler from requests coming from a C# System.Net.Mail.SMTPClient instance with credentials ([code](https://github.com/Cloud9Developer/Jellyfin-Newsletter-Plugin/blob/0c382440a75844a08e45a3dae13ed30d238d1eed/Jellyfin.Plugin.Newsletters/Emails/smtp.cs#L62-L65)).

I didn't understand the problem until I discovered with tcpdump that the login method is spelled "login" in lowercase:

and indeed [your code is not doing any case matching](https://github.com/aio-libs/aiosmtpd/blob/54b9643f232c92a0d651aa8dc0eddbd3af144174/aiosmtpd/smtp.py#L966-L969) on the said authentication method:

Would it be possible to transform method to uppercase before handling it?
```python
mechanism = mechanism.upper()
```

Thanks in advance.