Add PEP 723 inline script metadata support

[gilgamezh]

Requires #451 pointing to it until it's merged

Closes #423.

PEP 723 defines a standard # /// script comment block (TOML) for declaring a single-file script's dependencies and requires-python. It's implemented by pipx, pip-run and uv. This adds support to fades so it can run scripts written for other runners — and so scripts written for fades can run on them.

What it does

• dependencies: parsed from the block and merged like every other dependency source (inline # fades marks, docstrings, -r files, -d flags).
• requires-python: honored by selecting a suitable interpreter — the currently selected one (--python or fades' own) if it satisfies the specifier, otherwise an auto-discovered pythonX.Y on PATH, failing cleanly if none is available. An explicit --python that conflicts is reported rather than silently overridden, so the user stays in control.
• TOML parsing: stdlib tomllib on Python 3.11+, falling back to tomli on older versions (conditional dependency in setup.py).
• Robust errors: malformed metadata (bad TOML, invalid requirement string, bad/non-string requires-python, non-list dependencies, multiple script blocks) raises a clean FadesError with an explanatory log line instead of dumping a traceback.

Example:

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "requests<3",
#   "rich",
# ]
# ///

import requests
from rich.pretty import pprint

⚠️ Point for review: requires-python behavior

This goes beyond pipx and pip-run, which both ignore requires-python (only uv honors it, via its own interpreter management). Since fades has no interpreter management, the chosen behavior is:

• selected interpreter satisfies the specifier → use it;
• it doesn't, and no explicit --python → discover a matching pythonX.Y on PATH, else fail;
• it doesn't, and --python was given → fail (don't override the user's explicit choice).

Discovery is best-effort over PATH (python3.6–python3.29, python3, python). Happy to dial this back to validate-only if preferred.

Tests

New tests/test_parsing/test_pep723.py plus fixtures, interpreter-selection/version-probe tests in test_helpers.py, and a consolidate_dependencies test in test_main.py. Full suite green (./test), flake8 clean. Docs updated in README.rst and man/fades.1.

🤖 Generated with Claude Code

[gilgamezh]

Did run tests here because we only run them when target is master

https://github.com/PyAr/fades/actions/runs/27750679594

should be executed when we change the target branch