Last v2-compatible version, with v3 API

This is the last version of CCL that will be compatible with the v2 API. This version is also able to function under much of the basic v3 API, and will in fact throw warnings when used with the v2 API. This is intended as a way to soften the transition from v2 to v3. Users who want to benefit from it can migrate to this version first, fix their code until there are no more warnings, and then they'll be ready to use v3 proper. If you would rather stick to v2 for a while longer, without worrying about this, consider using the last pure v2 version (v2.7.0), which contains none of these warnings.

What's Changed

• Refactor pyccl under an abstract CCLObject. by @nikfilippas in #934
• Bump Python version to 3.8 by @nikfilippas in #1031
• RTD fix by @damonge in #1032
• Optimize HM autocorrelation power spectra by @nikfilippas in #891
• Fix incorrect Python version in CI by @nikfilippas in #1037
• Cleanup of conda-incubator & replace deprecated pkg_resources by @nikfilippas in #1045
• Fix error message for incorrect calls with unlock_instance by @nikfilippas in #1044
• Unfreeze option for physical_constants by @nikfilippas in #1050
• Implement separate __eq__ methods by @nikfilippas in #1033
• v2.final by @nikfilippas in #1038
• Docs fix by @damonge in #1081
• API restoration by @damonge in #1083
• IA profile v3 by @damonge in #1074
• Lightweight docs revamp for v2.last by @damonge in #1082
• Bug from sed by @damonge in #1086
• Changelog update by @damonge in #1089

Full Changelog: v2.7.0...v2.8.0