Implement hack to issue warnings during config

Once we can capture warnings during the config stage, we can
then get rid of this function

Related to #2891

src/_pytest/hookspec.py

@@ -526,7 +526,17 @@ def pytest_terminal_summary(terminalreporter, exitstatus):
 
 @hookspec(historic=True)
 def pytest_logwarning(message, code, nodeid, fslocation):
-    """ process a warning specified by a message, a code string,
+    """
+    .. deprecated:: 3.8
+
+        This hook is will stop working in a future release.
+
+        pytest no longer triggers this hook, but the
+        terminal writer still implements it to display warnings issued by
+        :meth:`_pytest.config.Config.warn` and :meth:`_pytest.nodes.Node.warn`. Calling those functions will be
+        an error in future releases.
+
+    process a warning specified by a message, a code string,
     a nodeid and fslocation (both of which may be None
     if the warning is not tied to a particular node/location).
 
@@ -538,7 +548,7 @@ def pytest_logwarning(message, code, nodeid, fslocation):
 @hookspec(historic=True)
 def pytest_warning_captured(warning_message, when, item):
     """
-    Process a warning captured by the internal pytest plugin.
+    Process a warning captured by the internal pytest warnings plugin.
 
     :param warnings.WarningMessage warning_message:
         The captured warning. This is the same object produced by :py:func:`warnings.catch_warnings`, and contains
@@ -546,6 +556,8 @@ def pytest_warning_captured(warning_message, when, item):
 
     :param str when:
         Indicates when the warning was captured. Possible values:
+
+        * ``"config"``: during pytest configuration/initialization stage.
         * ``"collect"``: during test collection.
         * ``"runtest"``: during test execution.