`debuglevel` is now a proper class attribute.
`debuglevel` is now a proper class attribute.

* Renamed `debuglevel` to `debug_level` to follow commonly used coding
style for class attributes.
* Made `set_debuglevel()` a classmethod.
* Changed `self.debuglevel` to `self.__class__.debug_level` everywhere.

Compatibility with code using this feature should be preserved.

Also, to change the debug level, we can now write:

    SMTP.debug_level = 1
    # or:
    server = SMTP()
    server.debug_level = 1
    # or:
    server.set_debuglevel(1)

hwmrocker/smtplibaio,Frzk/smtplibaio