mail: migrate to modern email.message instead of using old compat32 functionality

Before, the HTML part could be detected as pure 7-bit and sent as:
Content-Type: text/html; charset="us-ascii"
Content-Transfer-Encoding: 7bit

Now, it will use:
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: quoted-printable

That should work fine too.