Add support for pex3 lock subset.

[jsirois]

Closes #2411

[jsirois]

This was the only tricky bit. To correctly subset you need to consider extras but no other marker expressions. Luckily, the --elide-unused-requires-dists feature stabilized in https://github.com/pex-tool/pex/releases/tag/v2.25.2 included this machinery. The new integration tests cover this case.

[jsirois]

You also need to consider the markers faked for --style universal locks, namely "os_name", "platform_system" & "sys_platform" (here:

pex/pex/resolve/locker.py

Lines 568 to 583 in 16932ad

	# See https://www.python.org/dev/peps/pep-0508/#environment-markers for more about these values.
	_OS_NAME = {
	TargetSystem.LINUX: "posix",
	TargetSystem.MAC: "posix",
	TargetSystem.WINDOWS: "nt",
	}
	_PLATFORM_SYSTEM = {
	TargetSystem.LINUX: "Linux",
	TargetSystem.MAC: "Darwin",
	TargetSystem.WINDOWS: "Windows",
	}
	_SYS_PLATFORMS = {
	TargetSystem.LINUX: ("linux", "linux2"),
	TargetSystem.MAC: ("darwin",),
	TargetSystem.WINDOWS: ("win32",),
	}

) and "python_version" & "python_full_version" (here:

pex/pex/pip/foreign_platform/__init__.py

Lines 181 to 206 in 16932ad

	def patch_requires_python(
	requires_python, # type: Iterable[str]
	patches_dir=None, # type: Optional[str]
	):
	# type: (...) -> Patch
	"""N.B.: This Path exports Python version information in the `requires_python` module.
	Exports:
	+ PYTHON_FULL_VERSIONS: List[Tuple[int, int, int]]
	A sorted list of Python full versions compatible with the given `requires_python`.
	+ PYTHON_VERSIONS: List[Tuple[int, int]]
	A sorted list of Python versions compatible with the given `requires_python`.
	"""
	with TRACER.timed(
	"Calculating compatible python versions for {requires_python}".format(
	requires_python=requires_python
	)
	):
	python_full_versions = list(iter_compatible_versions(requires_python))
	with open(
	os.path.join(patches_dir or safe_mkdtemp(), "python_full_versions.json"), "w"
	) as fp:
	json.dump(python_full_versions, fp)
	return Patch.from_code_resource(
	__name__, "requires_python.py", _PEX_PYTHON_VERSIONS_FILE=fp.name
	)

). #2684 fixes this pretty huge oversight.

[huonw]

Very nice, thanks for picking up the feature request!