monkeypatch: remove `pkg_resources.fixup_namespace_packages` call in `syspath_prepend`

[bluetech]

monkeypatch.syspath_prepend calls it:

pytest/src/_pytest/monkeypatch.py

Lines 340 to 351 in 9d74e03

	def syspath_prepend(self, path) -> None:
	"""Prepend ``path`` to ``sys.path`` list of import locations."""
	if self._savesyspath is None:
	self._savesyspath = sys.path[:]
	sys.path.insert(0, str(path))
	# https://github.com/pypa/setuptools/blob/d8b901bc/docs/pkg_resources.txt#L162-L171
	# this is only needed when pkg_resources was already loaded by the namespace package
	if "pkg_resources" in sys.modules:
	from pkg_resources import fixup_namespace_packages
	fixup_namespace_packages(str(path))

This is needed for legacy namespace package support, which is now deprecated, along with the entire pkg_resources package.

I saw recently that zope packages which were the last holdouts I was familiar with had switched to new namespace package style, zope.event, zope.interface.

Let's start removing it. The main reason is that it requires us to have a setuptools dev dependency, ignore its deprecation warnings and keep it working.

Let's deprecate it and remove in pytest 10.