configparser: quadratic time in `ParsingError.combine()` when many lines fail to parse

[StanFromIreland]

Bug report

Bug description:

Found by OSS Fuzz in #501627726.

A simplified reproducer:

import time
import configparser

for N in (5000, 10000, 20000, 40000):
    s = '[*]\n' + '=\n' * N
    p = configparser.ConfigParser(strict=False)
    t0 = time.perf_counter()
    try:
        p.read_string(s)
    except configparser.ParsingError:
        pass
    print(f'{N}: {time.perf_counter() - t0}s')

produces:

5000: 0.5495022920076735s
10000: 2.0415573030040832s
20000: 7.589155077002943s
40000: 30.856201829999918s

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response