Support pylock.toml files in parse_requirements

[Copilot]

pip 26.1 added experimental support for pylock.toml lock files (PEP 751). This extends pip_api.parse_requirements to parse these files, gated behind a pip >= 26.1 version check.

Changes

• _is_pylock_file() — detects valid pylock filenames (pylock.toml or pylock.<name>.toml)
• _parse_pylock_requirements() — parses TOML content, extracting name, version, environment markers, and hashes (from wheels, sdist, archive entries); raises PipError when pip < 26.1
• parse_requirements() — routes pylock files through the new parser; works both when called directly and when referenced via -r pylock.toml inside a regular requirements file
• tests/data/pylock.toml — fixture with two packages including wheel hashes
• Tests — cover version gating, basic parsing, hash extraction, marker filtering, -r inclusion, and pylock.<name>.toml naming

Example

import pip_api

# Direct parsing
reqs = pip_api.parse_requirements("pylock.toml")
# {'foo': <Requirement('foo==1.2.3')>, 'bar': <Requirement('bar==2.0.0')>}

# Also works via -r reference in a requirements.txt
reqs = pip_api.parse_requirements("requirements.txt")
# where requirements.txt contains: -r pylock.toml

# Raises PipError if pip < 26.1
# PipError: Parsing pylock.toml files requires pip >= 26.1, but the current pip version is 24.0

[woodruffw]

Now that we've vendored packaging==26.2, I bet we could replace a lot of this with packaging.pylock's APIs, e.g. is_valid_pylock_path.

(We could probably also use PyLock.from_dict for stronger validation.)