When the content is not a string, EmailMessage.set_content requires a maintype
argument but not when it is a string. The maintype is then deduced using the
mimetypes module.
The Content-Language of content or alternate must be set using the headers
parameter instead of params which is for the Content-Type.
And finally the Content-Language must be set on the main message only if it
contains multiparts otherwise it is set twice.
Closes#14028
get_required_version does not need to use .dev0 suffix anymore.
The download URL must be empty for development version.
Source Code must point to Heptapod repository.
Foundation email is used as author email.
Cédric Krier