Deprecate raising unittest.SkipTest to skip tests during collection

It is not very clear why this code exists -- we are not running any
unittest or nose code during collection, and really these frameworks
don't have the concept of collection at all, and just raising these
exceptions at e.g. the module level would cause an error. So unless I'm
missing something, I don't think anyone is using this.

Deprecate it so we can eventually clear up this code and keep unittest
more tightly restricted to its plugin.

src/_pytest/deprecated.py

@@ -64,6 +64,11 @@ STRICT_OPTION = PytestDeprecationWarning(
 
 PRIVATE = PytestDeprecationWarning("A private pytest class or function was used.")
 
+UNITTEST_SKIP_DURING_COLLECTION = PytestDeprecationWarning(
+    "Raising unittest.SkipTest to skip tests during collection is deprecated. "
+    "Use pytest.skip() instead."
+)
+
 
 # You want to make some `__init__` or function "private".
 #

src/_pytest/runner.py

@@ -2,6 +2,7 @@
 import bdb
 import os
 import sys
+import warnings
 from typing import Callable
 from typing import cast
 from typing import Dict
@@ -27,6 +28,7 @@ from _pytest._code.code import TerminalRepr
 from _pytest.compat import final
 from _pytest.config.argparsing import Parser
 from _pytest.deprecated import check_ispytest
+from _pytest.deprecated import UNITTEST_SKIP_DURING_COLLECTION
 from _pytest.nodes import Collector
 from _pytest.nodes import Item
 from _pytest.nodes import Node
@@ -374,6 +376,11 @@ def pytest_make_collect_report(collector: Collector) -> CollectReport:
             # Type ignored because unittest is loaded dynamically.
             skip_exceptions.append(unittest.SkipTest)  # type: ignore
         if isinstance(call.excinfo.value, tuple(skip_exceptions)):
+            if unittest is not None and isinstance(
+                call.excinfo.value, unittest.SkipTest  # type: ignore[attr-defined]
+            ):
+                warnings.warn(UNITTEST_SKIP_DURING_COLLECTION, stacklevel=2)
+
             outcome = "skipped"
             r_ = collector._repr_failure_py(call.excinfo, "line")
             assert isinstance(r_, ExceptionChainRepr), repr(r_)