Prepare PyNWB 2.1.0

[rly]

Prepare PyNWB 2.1.0. Consider including #1432.

TODO:

• see steps below
• copy some infrastructure changes from HDMF repo
• check why test-validation CI fails

Prepare for release of PyNWB 2.1.0

Before merging:

• Major and minor releases: Update package versions in requirements.txt, requirements-dev.txt,
  requirements-doc.txt, requirements-min.txt, setup.py as needed
  See https://requires.io/github/NeurodataWithoutBorders/pynwb/requirements/?branch=dev
• Check legal file dates and information in Legal.txt, license.txt, README.rst, docs/source/conf.py,
  and any other locations as needed
• Update setup.py as needed
• Update README.rst as needed
• Update src/pynwb/nwb-schema submodule as needed. Check the version number and commit SHA manually
• Update changelog (set release date) in CHANGELOG.md and any other docs as needed
• Run tests locally including gallery tests and validation tests, and inspect all warnings and outputs
  (python test.py -v > out.txt)
• Test docs locally (make apidoc, make html)
• Push changes to this PR and make sure all PRs to be included in this release have been merged
• Check that the readthedocs build for this PR succeeds (build latest to pull the new branch, then activate and
  build docs for new branch): https://readthedocs.org/projects/pynwb/builds/

After merging:

1. Create release by following steps in docs/source/make_a_release.rst or use alias git pypi-release [tag] if set up
2. After the CI bot creates the new release (wait ~10 min), update the release notes on the
   GitHub releases page with the changelog
3. Check that the readthedocs "latest" and "stable" builds run and succeed
4. Update conda-forge/pynwb-feedstock with the latest version number
   and SHA256 retrieved from PyPI > PyNWB > Download Files > View hashes for the .tar.gz file. Re-render as needed

[codecov]

Codecov Report

Merging #1492 (930fe7c) into dev (4460a1b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1492   +/-   ##
=======================================
  Coverage   78.42%   78.42%           
=======================================
  Files          37       37           
  Lines        2777     2777           
  Branches      493      517   +24     
=======================================
  Hits         2178     2178           
  Misses        518      518           
  Partials       81       81           

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