Ranges are frozen by default in Ruby 3.0

[dleidert]

We are currently building and distributing your gem as Debian package. With Ruby 3.0 we see several test failures. They seem to be related to the fact that arrays are frozen by default in Ruby 3.0.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996297
https://people.debian.org/~kanashiro/ruby3.0/round2/builds/3/ruby-ice-nine/ruby-ice-nine_0.11.2-1+rebuild1633383280_amd64.build.txt
https://bugs.ruby-lang.org/issues/15504
https://github.com/ruby/ruby/pull/3583/files