[Ruby 3.0 support] Remove TRUE, FALSE, and NIL constants

[andrykonchin]

The PR fixes the following item:

[easy, pure ruby] TRUE/FALSE/NIL constants are no longer defined.

Related issue - #2453

[eregon]

Thanks!

[andrykonchin]

Not sure how to handle these failing specs in a proper way. Probably, specs should be run on CI with PRETEND_RUBY_VERSION=3.0.0.

Tests fail because the Ruby 2.7 behavior is tested. But here in Ruby 3.0 we have breaking changes. So it's quite expected that new behavior causes these failures.

Example of failed spec:

    ruby_version_is ""..."3.0" do
      it "includes FALSE" do
        Object.const_defined?(:FALSE).should == true
        -> { FALSE }.should complain(/constant ::FALSE is deprecated/)
      end
    end

[eregon]

@andrykonchin I'll take care of it. I'll try to merge the Ruby 3 import PR first, which bumps the RUBY_VERSION and therefore solves this.