I'm trying to wait for the final release until we can use Azure to build wheels for Python 3.9. The relevant images on Azure were expected early this week. Once they're available, then we'd ideally want Numpy to release wheels for Python 3.9, so that we can build h5py using those. If this takes too much longer, I could release 3.0 without wheels for Python 3.9, and then do another release soon after. But it's supposed to be ready any day now.
I'm also planning to drop Python 3.6 wheels at the same time as adding 3.9, to keep the CI times reasonable. NEP 29 recommends dropping 3.6 from June 2020. It would still be possible to install h5py 3.0 from source in Python 3.6, but there wouldn't be a wheel. If this is going to be a big problem, we could keep 3.6 wheels for this release, but we'll be dropping 3.6 support soon afterwards.