Improve `PyBytes_FromStringAndSize` test coverage

[sobolevn]

There's a fast-path in PyBytes_FromStringAndSize that is not covered:

cpython/Objects/bytesobject.c

Lines 120 to 124 in 8549559

	if (size == 1 && str != NULL) {
	op = CHARACTER(*str & 255);
	assert(_Py_IsImmortal(op));
	return (PyObject *)op;
	}

Here:

cpython/Objects/bytesobject.c

Lines 111 to 124 in 8549559

	PyObject *
	PyBytes_FromStringAndSize(const char *str, Py_ssize_t size)
	{
	PyBytesObject *op;
	if (size < 0) {
	PyErr_SetString(PyExc_SystemError,
	"Negative size passed to PyBytes_FromStringAndSize");
	return NULL;
	}
	if (size == 1 && str != NULL) {
	op = CHARACTER(*str & 255);
	assert(_Py_IsImmortal(op));
	return (PyObject *)op;
	}

We don't have test cases for it:

cpython/Lib/test/test_capi/test_bytes.py

Lines 49 to 65 in 8549559

	def test_fromstringandsize(self):
	# Test PyBytes_FromStringAndSize()
	fromstringandsize = _testlimitedcapi.bytes_fromstringandsize
	self.assertEqual(fromstringandsize(b'abc'), b'abc')
	self.assertEqual(fromstringandsize(b'abc', 2), b'ab')
	self.assertEqual(fromstringandsize(b'abc\0def'), b'abc\0def')
	self.assertEqual(fromstringandsize(b'', 0), b'')
	self.assertEqual(fromstringandsize(NULL, 0), b'')
	self.assertEqual(len(fromstringandsize(NULL, 3)), 3)
	self.assertRaises((MemoryError, OverflowError),
	fromstringandsize, NULL, PY_SSIZE_T_MAX)
	self.assertRaises(SystemError, fromstringandsize, b'abc', -1)
	self.assertRaises(SystemError, fromstringandsize, b'abc', PY_SSIZE_T_MIN)
	self.assertRaises(SystemError, fromstringandsize, NULL, -1)
	self.assertRaises(SystemError, fromstringandsize, NULL, PY_SSIZE_T_MIN)

I propose adding:

self.assertEqual(fromstringandsize(b'a'), b'a')
self.assertEqual(fromstringandsize(b'a', 1), b'a')

So we would have better C coverage stats. It won't hurt in any case, even if this path is removed some day.

Linked PRs

• gh-121842: Improve coverage of PyBytes_FromStringAndSize #121843
• [3.13] gh-121842: Improve coverage of PyBytes_FromStringAndSize (GH-121843) #121893
• [3.12] gh-121842: Improve coverage of PyBytes_FromStringAndSize (GH-121843) #121894