We remove the safeguards that were added in `Lib/test/test_capi/test_codecs.py`
since they are now redundant (see 32e07fd377
for additional context).
Indeed, the codecs handlers now correctly handle the `start` and `end` positions
of `UnicodeError` objects and thus should not crash.
This fixes how `PyCodec_BackslashReplaceErrors` handles the `start` and `end`
attributes of `UnicodeError` objects via the `_PyUnicodeError_GetParams` helper.
This fixes how `PyCodec_XMLCharRefReplaceErrors` handles the `start` and `end`
attributes of `UnicodeError` objects via the `_PyUnicodeError_GetParams` helper.
Split unicode.c tests of _testcapi into two parts: limited C API
tests in _testlimitedcapi and non-limited C API tests in _testcapi.
Update test_codecs.
Bénédikt Tran