os.add_dll_directory required on CPython 3.8 and later

[zooba]

I haven't done the historical work, but at some point this code below was added/changed.

It's not mysterious - conda-forge has a patch to disable add_dll_directory and reenable finding DLLs on PATH. Because regular CPython does not have this patch, this change won't work in a range of scenarios where PATH is ignored.

So this needs the add_dll_directory call as well (or ideally, conda-forge would improve their patch, but I think that's unlikely right now).

pyzmq/zmq/__init__.py

Lines 65 to 96 in b33873c

	def _libs_on_path():
	"""context manager for libs directory on $PATH
	Works around mysterious issue where os.add_dll_directory
	does not resolve imports (conda-forge Python >= 3.8)
	"""
	if not sys.platform.startswith("win"):
	yield
	return
	libs_dir = os.path.abspath(
	os.path.join(
	os.path.dirname(__file__),
	os.pardir,
	"pyzmq.libs",
	)
	)
	if not os.path.exists(libs_dir):
	# no bundled libs
	yield
	return
	path_before = os.environ.get("PATH")
	try:
	os.environ["PATH"] = os.pathsep.join([path_before or "", libs_dir])
	yield
	finally:
	if path_before is None:
	os.environ.pop("PATH")
	else:
	os.environ["PATH"] = path_before

[zooba]

I see there's code higher up that ought to be doing the same thing. That makes this less urgent, but it does make this file pretty messy.

My other issues when I noticed this are covered by #1460

[minrk]

There are two ways libzmq can be 'bundled' with pyzmq:

1. externally built libzmq vendored in pyzmq.libs by delvewheel/auditwheel/delocate, in which case it is loaded as a regular DLL dependency of the backend at import time, and
2. libzmq built by pyzmq itself as a Python Extension, in which case it's loaded via import zmq._libzmq before importing the backend

Option 2 was how we did it on Windows for a long time time, and what happens when pyzmq is built from source with libzmq unavailable on the system (~never on Windows). 1. is generally preferable, and, thanks to delvewheel, is how we do it now in almost all cases (I couldn't get it to work on Windows + PyPy for some reason, so that's the only remaining case where a pyzmq wheel bundles libzmq as an Extension).

Whatever advice you have for finding DLLs in pyzmq.libs that works consistently across Python versions and implementations would be welcome!