bpo-27397: Make email module properly handle invalid-length base64 strings (GH-7583...

bpo-27397: Make email module properly handle invalid-length base64 strings (GH-7583) (GH-7664)

When attempting to base64-decode a payload of invalid length (1 mod 4),
properly recognize and handle it.  The given data will be returned as-is,
i.e. not decoded, along with a new defect, InvalidBase64LengthDefect.
(cherry picked from commit c3f55be7dd012b7e92901627d0b31c21e983ccb4)

Co-authored-by: Tal Einat <taleinat+github@gmail.com>