Support Ruby 3.0

[skalee]

This pull request adds compatibility with Ruby 3.0, which has been released recently.

The only important change is that REXML has been specified as a runtime dependency. This is necessary because REXML is now a bundled gem (see https://stdgems.org/). Version constraint for this gem (~> 3.0) is relaxed enough to support REXML provided in Ruby 2.6 and 2.7.

[inukshuk]

Thanks! We can make the dependency optional even, since this is only used to convert to BibTeXML.

[skalee]

It's a bundled gem in Ruby 3 and a default gem in Ruby 2.6-2.7. That means that all these Rubies have REXML gem pre-installed.

[skalee]

Placing REXML in Gemfile only as in b002b79 is not a good idea IMO. Tests will pass, locally it will work, but software which depends on this gem will break unless developers explicitly specify REXML gem in the Gemfile or gemspec themselves.

[inukshuk]

It will break only if they use the BibTeXML conversion though (or did I miss a require?) and only if they do not have the Gem installed (which, as you've pointed out should be pre-installed currently). In this case it's just like other optional dependencies (like the old rdf gem) are treated.

[inukshuk]

I did miss one require. They should all be allowed to fail.

[inukshuk]

The idea is that you have to specify REXML as a dependency if you use the BibTeXML conversion -- if you don't, that converter will not be available and some to_xml methods will fail if you do use them. Is that OK with you? We can also bump the major version for this.

[skalee]

Well, it's all up to you. ;) Do whatever you consider the best for this project, I guess you'll have to add some installation notes in README or something. Sure I can simply add REXML as a runtime dependency to my gem, no worries here.

[tmaier]

Hi @skalee,

I am still collecting the consents for the license change, which I propose at #154. It would be really great, if you could consent to it as well, because it would allow moving on.

Thank you and kind regards

Tobias