Match CPython's str subscript TypeError message

[kim-jaedeok]

Summary

Make str subscripting with a non-integer index raise the same TypeError
message as CPython, and drop the expectedFailure marker on
string_tests.test_subscript.

Before / after ('abc'['def']):

	message
CPython 3.14	string indices must be integers, not 'str'
RustPython (before)	str indices must be integers or slices or classes that override __index__ operator, not 'str'
RustPython (after)	string indices must be integers, not 'str'

Only the error message differed — indexing itself (int / negative / slice)
already behaved correctly. CPython gives str its own wording in
unicode_subscript (see python/cpython#88276, fixed in 3.11), distinct from the
shared sequence message used by list/tuple/bytes.

Approach

• Add SequenceIndex::try_from_borrowed_object_with_err, which keeps the shared
  index-parsing logic but lets the caller supply the TypeError raised for a
  non-index object.
• try_from_borrowed_object now delegates to it with the existing wording, so
  every other sequence caller (list/tuple/bytes/bytearray/array/mmap/
  struct-sequence) is byte-for-byte unchanged.
• str::_getitem — the single path shared by __getitem__ and the mapping
  subscript slot — supplies the CPython-accurate message.

Scoped intentionally to str only: CPython declined to unify the sequence
messages (python/cpython#75731, rejected), so the other types' wording is left
as-is.

Testing

• test_str / test_userstring test_subscript now pass (marker removed).
• Byte-for-byte match with CPython 3.14.6 for str/object/None/float
  indices, and for UserString as receiver.
• list/tuple/bytes messages verified unchanged; rustpython-vm and
  rustpython-codegen unit tests pass.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Aligns RustPython’s str subscripting TypeError wording with CPython by allowing str to supply a custom non-index error message while keeping the shared sequence index parsing logic unchanged for all other sequence types.

Changes:

• Added SequenceIndex::try_from_borrowed_object_with_err to allow callers to customize the TypeError emitted for non-index objects.
• Updated str subscripting (PyStr::_getitem) to emit CPython’s "string indices must be integers, not '...'" message.
• Removed the @unittest.expectedFailure marker from Lib/test/string_tests.py::test_subscript now that the test passes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Lib/test/string_tests.py	Removes expectedFailure for test_subscript now that the error message matches CPython.
crates/vm/src/sliceable.rs	Introduces an alternate SequenceIndex constructor that accepts a caller-provided TypeError for non-index objects.
crates/vm/src/builtins/str.rs	Uses the new constructor to produce CPython-accurate str subscript TypeError wording.

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[kim-jaedeok]

Superseded by the upstream PR: RustPython#8319