Update docs and version strings for version 1.2.1

[rly]

Prepare for release of hdmf-common-schema 1.2.1 on November 4, 2020.

Pending merge of https://github.com/hdmf-dev/hdmf-common-schema/pull/41

• [x] Update this branch

Before merging:

• [x] Update requirements versions as needed
• [x] Update legal file dates and information in Legal.txt, license.txt, README.rst, docs/source/conf.py, and any other locations as needed
• [x] Update README.md as needed
• [x] Update the version string in docs/source/conf.py and common/namespace.yaml (remove "-alpha" suffix)
• [x] Update docs/source/conf.py as needed
• [x] Update release notes (set release date) in docs/source/format_release_notes.rst and any other docs as needed
• [x] Test docs locally (make fulldoc).
• [x] Push changes to this PR and make sure all PRs to be included in this release have been merged
• [x] Point the HDMF submodule to this branch in the HDMF branch corresponding to this schema version and run HDMF tests
• [x] 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/hdmf-common-schema/builds/

After merging:

1. Create a new git tag. Pull the latest master branch locally, run git tag [version] --sign, copy and paste the release notes into the tag message, and run git push --tags.
2. On the GitHub tags page page, click "Create release" to create a release from the new tag. Copy and paste the release notes into the release message and update the title to the version string.
3. Check that the readthedocs "latest" and "stable" builds run and succeed

See https://hdmf-common-schema.readthedocs.io/en/latest/software_process.html for more details.

[rly]

The generated documentation does not list CSRMatrix or SimpleMultiContainer as children of Container for some reason. I'll work on resolving that today. I recommend we hold on merging this until that is resolved. It is probably a bug in hdmf_docutils.

[rly]

Oh, I know the issue. hdmf_docutils builds documentation based on the schema bundled in the latest release of hdmf instead of the local schema. https://github.com/hdmf-dev/hdmf-docutils/issues/60

[rly]

The issue with the wrong schema being loaded is resolved in this PR as well.