Drop py36 support, update CI, reqs, tests for rc/2.0.0

[rly]

• Drop py36 support, update CI and requirements
• Add py39 support
• Fix numpy deprecation errors in tests
• Fix validation test errors
• Fix dtype conversion and deprecation warnings in tests
• Fix backward compatibility test warnings

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
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• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[rly]

gallery39 tests fail because the allensdk does not yet support python 3.9, so CI for PR will run gallery38 tests instead of gallery39 tests for now

[oruebel]

Looks good

[rly]

The gallery38 error is due to an incompatibility between allensdk and h5py 3 (Dataset.value is deprecated). I believe this will be addressed soon as they work on adding h5py 3 and py39 support after pynwb/hdmf adds it.