Avoid breaking change in psych v4 in ruby v3.1

ruby 3.1 was shipped with psych v4 which included a breaking change [1]
that was mentioned in the ruby v3.1 release notes [2] (see the "Other
notable changes since 3.0" section).

This commit constrains psych to < v4. I haven't investigated, but I
suspect the code using psych is in rubocop, so we might be able to avoid
adding this constraint if we update to a more recent version of rubocop.

[1]: https://bugs.ruby-lang.org/issues/17866
[2]: https://www.ruby-lang.org/en/news/2021/12/25/ruby-3-1-0-released/

mocha.gemspec

@@ -41,6 +41,12 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   end
 
   s.add_development_dependency('introspection', '~> 0.0.1')
+
+  # Avoid breaking change in psych v4 (https://bugs.ruby-lang.org/issues/17866)
+  if RUBY_VERSION >= '3.1.0'
+    s.add_development_dependency('psych', '< 4')
+  end
+
   if RUBY_VERSION >= '2.2.0'
     # No test libraries in standard library
     s.add_development_dependency('minitest')