Support both Psych 3 and 4

[okkez]

Ruby 3.1.2 includes Psych 4.0.4 by default. So I met some breaking changes I use this gem with Ruby 3.1.2.

I've added the compat layer for this.

See also:

• https://bugs.ruby-lang.org/issues/17866
• https://github.com/rails/rails/pull/42257/files

[okkez]

@joker1007 Can you take a look?