Move some mostly redundant CI from on push to nightly

[rly]

Motivation

Our CI involves too many jobs -- it takes a long time for all the jobs to complete, especially when trying to merge successive PRs. Some of these jobs can be moved to a nightly build rather than an on-every-push build.

With this PR, CI will consist of:
CircleCI:

• flake8
• python38 (3.8 with pip install pinned requirements)
• python36-min-req (3.6 with pip install minimal requirements)
• miniconda38 (3.8 on conda with pinned requirements)
• miniconda36 (3.6 on conda with pinned requirements)
• gallery38 (gallery/example tests...)
• gallery36-min-req
• pynwb-dev-python38 (git pull dev branch of pynwb and run tests)

Azure Pipelines:

• macOS-py3.8
• Windows-py3.8
• macOS-py3.6-min-req
• Windows-py3.6-min-req

I changed the required CI for a PR merge to be just:

• circleci/flake8
• circleci/python36-min-req
• circleci/python38
• azure/windows-py3.8

The 'deploy-release' job that creates the pre-releases will move from every time a commit is made to dev to a nightly build.

Nightly build includes testing on every configuration including Python 3.9.

This will cut on-push CI down to 12 jobs + 3 for code coverage, down from 25 + 3 and reduce the CI time on a commit to dev.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• 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.