Add bootstrapping and isolation of development Python versions (#1105)

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.

scripts/scenarios/update.py

@@ -39,14 +39,13 @@ Requirements:
 
 import json
 import shutil
-import os
 import subprocess
 import sys
 import textwrap
 from pathlib import Path
 
 
-PACKSE_COMMIT = "78f34eec66acfba9c723285764dc1f4b841f4961"
+PACKSE_COMMIT = "e944cb4c8f5d68457d0462ee19106509f63b8d34"
 TOOL_ROOT = Path(__file__).parent
 TEMPLATES = TOOL_ROOT / "templates"
 INSTALL_TEMPLATE = TEMPLATES / "install.mustache"
@@ -149,6 +148,8 @@ print("Loading scenario metadata...", file=sys.stderr)
 data = json.loads(
     subprocess.check_output(
         [
+            sys.executable,
+            "-m",
             "packse",
             "inspect",
             "--short-names",