CI is broken on ruby-head

[eregon]

See https://github.com/ruby/pp/actions/runs/7489634039/job/20386628987

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Failure: test_lasgn_literal(PPTestModule::PPAbstractSyntaxTree)
/home/runner/work/pp/pp/test/test_pp.rb:235:in `test_lasgn_literal'
     232:     def test_lasgn_literal
     233:       ast = AST.parse("_=1")
     234:       expected = "(SCOPE@1:0-1:3 tbl: [:_] args: nil body: (LASGN@1:0-1:3 :_ (LIT@1:2-1:3 1)))"
  => 235:       assert_equal(expected, PP.singleline_pp(ast, ''.dup), ast)
     236:     end
     237:   end
     238: end
#<RubyVM::AbstractSyntaxTree::Node:0x00007f8b9029b4e0>
<"(SCOPE@1:0-1:3 tbl: [:_] args: nil body: (LASGN@1:0-1:3 :_ (LIT@1:2-1:3 1)))"> expected but was
<"(SCOPE@1:0-1:3 tbl: [:_] args: nil body: (LASGN@1:0-1:3 :_ (INTEGER@1:2-1:3 1)))">

diff:
? (SCOPE@1:0-1:3 tbl: [:_] args: nil body: (LASGN@1:0-1:3 :_ (LINTEGER@1:2-1:3 1)))
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Probably not ideal to depend on unstable/experimental RubyVM::AbstractSyntaxTree in pp's CI.

[hsbt]

The current HEAD is working fine.