[draft] Support for Python 3.10

[kharmabum]

Opening this PR to start a conversation about upgrading Escher dependencies to support a Python 3.10 environment.

Changes

• Add reference requirements.txt (facilitates testing/development by enabling removing of pytest from install_requires)
• Migrate test dependencies to extra_requires
• Upgrade Jinja2, jsonschema, and ipywidgets
• Update .gitignore
• Update README
• Add explicit npm version dependency via .nvmrc