Recommend setting upper pin on hatch-requirements-txt

[maresb]

In build-system.requires I think we should pin hatch-requirements-txt<2 in order to leverage semver so that we can publish breaking changes (like deprecation). But we are in this weird <1.0 pre-release realm, so maybe it would make sense to also do a 1.0 release? I don't really have strong feelings, but I wanted to float the idea in the context of the changes being currently proposed.