In Ruby 3.1 those gems were dropped from the stdlib, so they need to be
explicitly installed. Mail should be doing this for us, but since it
cares about Ruby < 2.6, and those gems can't be installed there, they
can't add them to the gemspec without dropping support to old rubies.
Since we don't care about Ruby < 2.7, we can just require them in all
frameworks that use mail.