Add Ruby 3.1 support

Add net-smtp as a dependency for Ruby 3.1 compatibility

It was recently removed from the default gems:

  - https://bugs.ruby-lang.org/issues/17873
  - https://github.com/ruby/ruby/pull/4530

Also add net-imap and net-pop to the gemspec

This dependencies introduces `strscan` as a subdependency, but JRuby
only has prerelease support for it. Make sure the prerelease is
properly picked up by our tests by adding it to the `Gemfile`.

.github/workflows/test.yml

@@ -23,6 +23,8 @@ jobs:
            timeout: 5
          - ruby: '3.0'
            timeout: 5
+         - ruby: 3.1
+           timeout: 5
          - ruby: truffleruby-head
            timeout: 50
          - ruby: jruby

Gemfile

@@ -13,3 +13,5 @@ gem 'jruby-openssl', :platforms => :jruby
 gem 'mini_mime'
 
 gem 'byebug', :platforms => :mri
+
+gem "strscan", ">= 3.0.2.pre1"

mail.gemspec

@@ -16,6 +16,9 @@ Gem::Specification.new do |s|
   s.required_ruby_version = ">= 2.5"
 
   s.add_dependency('mini_mime', '>= 0.1.1')
+  s.add_dependency('net-smtp')
+  s.add_dependency('net-imap')
+  s.add_dependency('net-pop')
 
   s.add_development_dependency('bundler', '>= 1.0.3')
   s.add_development_dependency('rake', '> 0.8.7')