Update mkdocstrings and mkdocstrings-python, pin griffe, restructure poetry dependencies

[glennmatthews]

Closes: #N/A

Problems:

• ReadTheDocs build was broken the last couple of days because we don't pin our griffe dependency and a new version of griffe was released that breaks compatibility with the older versions of mkdocstrings and mkdocstrings-python we were pinned to.
• [tool.poetry.dev-dependencies] in pyproject.toml has been deprecated by Poetry since version 1.2.0 in favor of [tool.poetry.group.dev.dependencies], and newer versions of Poetry don't add things to the deprecated section any more, requiring manual cleanup of pyproject.toml.

Solutions

• Put in the time to restructure pyproject.toml to match Poetry's newer expectations. I've split our "dev" dependencies into the sub-groups of "dev", "docs", "linting", and "testing", all of which are still installed by default by Poetry but could in future be installed separately or together as best suits needs.
• Pin griffe docs dependency in both pyproject.toml and docs/requirements.txt to the latest version that still supports Python 3.7. (The new version just released, 0.31, drops support for 3.7, so we can't use it just yet)
• Update mkdocstrings and mkdocstrings-python docs dependencies in both pyproject.toml and docs/requirements.txt to latest versions for compatibility with this version of griffe.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• n/a Attached Screenshots, Payload Example
• n/a Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design

[HanlinMiao]

Should factory-boy be under testing?
I guess it is used for more than just testing.

[glennmatthews]

Certainly could be - I recognize that these categories are fairly arbitrary. :-)

[HanlinMiao]

LGTM!