0.2.0 postmortem ☠️

[nrbgt]

After #18, we noted a few things:

• should generate SHA256SUMS during CI
• the release verification was based on notebooks having been linted, which is pretty slow
• starting to get some package drift
• need to start a new version and changelog

This PR:

• conservatively bumps the versions to 0.2.1, in case we find bugs-what-need-fixen
• adds a build:hash task to generate SHA256SUMS file
  • skipped on windows, shells out to sha256sum
• moves the nbconvert output to build
• reworks some of the dependencies
• resolves the envs

Happy to hear any other things we could capture in the process (short of automating release, which isn't that important just yet).

Out of scope is going to a reproducible build system (e.g. flit) as there's no good solution for the packaging jumping jacks required for #6... might renew a push (several levels) upstream on this, as it concerns me.

[dfreeman06]

Thanks. Good catch on the dangling ipyelk/tools/__init__.py. I was confused for too long when i saw in the PR that the file was deleted.