Ruby 3.0 deprecated rb_cData with the intention to remove it in release 3.1.
This commit changes references of rb_cData to rb_cObject. This was done so we
can support distributions that package Ruby 3.0, such as Fedora 34.
We also need to call rb_undef_alloc_func because we're no longer deriving from
rb_cData. This prevents unnecessary allocations.
See:
https://docs.ruby-lang.org/en/3.0.0/doc/extension_rdoc.html
"It is recommended that klass derives from a special class called Data
(rb_cData) but not from Object or other ordinal classes. If it doesn't,
you have to call rb_undef_alloc_func(klass)."
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