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
  - ruby/ruby#4530

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

JRuby already has prerelease support for `io-wait` as a gem, and an
almost ready PR to also support `strscan` as a gem, dependencies
introduced by this change. Adding both to the Gemfile gets `bundle
install` to succeed, although specs fail later because Bundler does not
seem to support pointing to a java extension gem through git.

.github/workflows/test.yml

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

Gemfile

@@ -15,3 +15,9 @@ gem 'mini_mime'
 gem 'byebug', :platforms => :mri
 
 gem "psych", "< 4"
+
+gem "io-wait", ">= 0.1.0.a"
+
+if RUBY_PLATFORM == "java"
+  gem "strscan", github: "https://github.com/ruby/strscan/pull/25"
+end

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')