deprecate marker based hook metadata

[RonnyPfannschmidt]

followup to #891

while implementing i noticed we support attribute/mark based hook metadata just in pytest,
we should deprecate that custom parsing and rely entirely on pluggy markers

[blueyed]

Just for reference: instead of @pytest.mark.hookwrapper @pytest.hookimpl(hookwrapper=True) should be used.
Related code:

pytest/src/_pytest/config/__init__.py

Lines 259 to 287 in 19035f4

	def parse_hookimpl_opts(self, plugin, name):
	# pytest hooks are always prefixed with pytest_
	# so we avoid accessing possibly non-readable attributes
	# (see issue #1073)
	if not name.startswith("pytest_"):
	return
	# ignore names which can not be hooks
	if name == "pytest_plugins":
	return
	method = getattr(plugin, name)
	opts = super(PytestPluginManager, self).parse_hookimpl_opts(plugin, name)
	# consider only actual functions for hooks (#3775)
	if not inspect.isroutine(method):
	return
	# collect unmarked hooks as long as they have the `pytest_' prefix
	if opts is None and name.startswith("pytest_"):
	opts = {}
	if opts is not None:
	# TODO: DeprecationWarning, people should use hookimpl
	# https://github.com/pytest-dev/pytest/issues/4562
	known_marks = {m.name for m in getattr(method, "pytestmark", [])}
	for name in ("tryfirst", "trylast", "optionalhook", "hookwrapper"):
	opts.setdefault(name, hasattr(method, name) or name in known_marks)
	return opts

(I've just came along about a plugin using @pytest.mark.hookwrapper and looked it up)

[bluetech]

I added this to the 6.1 milestone: hopefully we can start emitting deprecation warnings in 6.x. However the @pytest.mark.{tryfirst,trylast,hookwrapper,optionalhook} hooks are very commonly used in external plugins so not sure when it would make sense to completely remove.

[RonnyPfannschmidt]

@nicoddemus while investigating i noted, that it seems like we dont have a good way to issue/record warnings with specific locations at that point in time,

this seems hard to correctly fix at first glance, so i will defer it until i can spend some more time with the issue at hand