config.types: better ValidatedAttribute(parse=bool) deprecation warnings

This will fire only once, when the setting object is initialized.

Attaching the deprecation to the boolean helper functions seemed to fire
every time a ValidatedAttribute in bool mode was accessed. Very spammy.

sopel/config/types.py

@@ -30,8 +30,9 @@
 import os.path
 import re
 import sys
+import traceback
 
-from sopel.tools import deprecated, get_input
+from sopel.tools import get_input, stderr
 
 if sys.version_info.major >= 3:
     unicode = str
@@ -264,11 +265,6 @@ def __delete__(self, instance):
         instance._parser.remove_option(instance._section_name, self.name)
 
 
-@deprecated(
-    reason="Use BooleanAttribute instead of ValidatedAttribute with bool as parser",
-    version='7.1',
-    removed_in='9.0',
-)
 def _parse_boolean(value):
     if value is True or value == 1:
         return value
@@ -277,11 +273,6 @@ def _parse_boolean(value):
     return bool(value)
 
 
-@deprecated(
-    reason="Use BooleanAttribute instead of ValidatedAttribute with bool as parser",
-    version='7.1',
-    removed_in='9.0',
-)
 def _serialize_boolean(value):
     return 'true' if _parse_boolean(value) else 'false'
 
@@ -309,10 +300,24 @@ def __init__(self,
                  is_secret=False):
         super(ValidatedAttribute, self).__init__(
             name, default=default, is_secret=is_secret)
+
         if parse == bool:
             parse = _parse_boolean
             if not serialize or serialize == bool:
                 serialize = _serialize_boolean
+
+            # deprecation warning
+            # can't use tools.deprecated in this case, unfortunately
+            # we need this to be conditional
+            msg = (
+                'Deprecated since 7.1, '
+                'will be removed in 9.0: '
+                'Use BooleanAttribute instead of ValidatedAttribute with parse=bool')
+            stderr(msg)
+            # Only display the last stack frame
+            trace = traceback.extract_stack()
+            stderr(traceback.format_list(trace[:-1])[-1][:-1])
+
         self.parse = parse or self.parse
         self.serialize = serialize or self.serialize