Replaces https://github.com/astral-sh/puffin/pull/1068 and #1070 which
were more complicated than I wanted.
- Introduces a `.python-versions` file which defines the Python versions
needed for development
- Adds a Bash script at `scripts/bootstrap/install` which installs the
required Python versions from `python-build-standalone` to `./bin`
- Checks in a `versions.json` file with metadata about available
versions on each platform and a `fetch-version` Python script derived
from `rye` for updating the versions
- Updates CI to use these Python builds instead of the `setup-python`
action
- Updates to the latest packse scenarios which require Python 3.8+
instead of 3.7+ since we cannot use 3.7 anymore and includes new test
coverage of patch Python version requests
- Adds a `PUFFIN_PYTHON_PATH` variable to prevent lookup of system
Python versions for isolation during development
Tested on Linux (via CI) and macOS (locally) — presumably it will be a
bit more complicated to do proper Windows support.
