gh-156187: Fix the warning stacklevel inside a nested set operand

_parse_charset() derives the stacklevel of its FutureWarning from
nested, and the nested-operand path inserts two frames, _parse_operand()
and the inner _parse_charset(), while advancing nested by one.  The
warning was therefore reported against a frame inside the re package
instead of the caller, and further off the deeper the nesting.

Author: fedonman

Lib/re/_parser.py

@@ -606,7 +606,7 @@ def addmember(code):
     if allow_nested and sourcematch("["):
         # A nested set after an operator is the whole operand, used as-is (not
         # wrapped in a group); it cannot be combined with loose members.
-        compound = _parse_charset(source, state, nested + 1)
+        compound = _parse_charset(source, state, nested + 2)
     while True:
         this = sourceget()
         if this is None:

Lib/test/test_re.py

@@ -1537,6 +1537,10 @@ def test_set_operations(self):
         with warnings.catch_warnings():
             warnings.simplefilter('error', FutureWarning)
             re.compile(r'[a-z--[aeiou]]')
+        # A reserved construct inside a nested operand warns against the caller.
+        with self.assertWarnsRegex(FutureWarning, 'Possible nested set ') as w:
+            re.compile(r'[a--[[b]]]')
+        self.assertEqual(w.filename, __file__)
 
         # Set union  A||B == A or B (an explicit form of [AB]); flat operands
         # merge into one charset, otherwise the operations are alternated.
@@ -1557,6 +1561,10 @@ def test_set_operations(self):
             self.assertEqual(re.findall(r'[\d~~1]', s), list('0123456789~'))
         self.assertEqual(w.filename, __file__)
         self.assertEqual(re.findall(r'[~~1]', s), list('1~'))
+        with self.assertWarnsRegex(FutureWarning,
+                                   'Possible set symmetric difference ') as w:
+            re.compile(r'[a||[c~~d]]')
+        self.assertEqual(w.filename, __file__)
 
     def test_search_coverage(self):
         self.assertEqual(re.search(r"\s(b)", " b").group(1), "b")