Fix Read the Docs build environment

[daniellovell]

Pin the poetry version to 1.5.1, which is known to work well with ReadTheDocs Explicitly install virtualenv 20.23.1, which doesn't have the issue with the 'free_threaded' attribute

Related PRs / Issues
Ideally fixes the failing check in #886
(tangential to the contents of that PR)

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• Change to core Hammer
• Change to a Hammer plugin
• Other

Contributor Checklist:

• Did you set master as the base branch?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you update the poetry.lock file if you updated the requirements in pyproject.toml?
• (If applicable) Did you add a unit test demonstrating the PR?
• (If applicable) Did you run this through the e2e integration tests?
• (If applicable) Did you update the submodules in e2e/ if this feature depends on updated plugins?