config.types: add deprecation warnings for ValidatedAttribute with bool

The boolean helper functions seem to be the best place for marking this
special case of ValidatedAttribute as deprecated, since they are ONLY
called when the special logic gets invoked.

sopel/config/types.py

@@ -31,7 +31,7 @@
 import re
 import sys
 
-from sopel.tools import get_input
+from sopel.tools import deprecated, get_input
 
 if sys.version_info.major >= 3:
     unicode = str
@@ -264,6 +264,11 @@ 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
@@ -272,6 +277,11 @@ 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'