[3.12] gh-113785: csv: fields starting with escapechar are not quoted (GH-122110)

[miss-islington]

(cherry picked from commit a3327db)

Co-authored-by: Mikołaj Kuranowski mkuranowski@gmail.com

• Issue: CSV reader inconsistent with combination of QUOTE_NONNUMERIC and escapechar #113785

[hugovk]

@serhiy-storchaka This didn't automerge because test_csv is failing on the CI:

======================================================================
FAIL: test_read_quoting (test.test_csv.Test_Csv.test_read_quoting)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/test_csv.py", line 428, in test_read_quoting
    self._read_test(['1\\.5,\\.5,.5'], [[1.5, 0.5, 0.5]],
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/test_csv.py", line 354, in _read_test
    self.assertEqual(result, expect)
AssertionError: Lists differ: [[1.5, '.5', 0.5]] != [[1.5, 0.5, 0.5]]

First differing element 0:
[1.5, '.5', 0.5]
[1.5, 0.5, 0.5]

- [[1.5, '.5', 0.5]]
?        ^  -

+ [[1.5, 0.5, 0.5]]
?        ^

[serhiy-storchaka]

Oh yes. It depended on #113732. It is possible to resolve this issue without touching that change. But It may be safer to not backport this change to 3.12.

[serhiy-storchaka]

Oh, I did not mean to merge it now. Well, it doesn't matter.