Remove leftover bytes support from _elementtree

[serhiy-storchaka]

Bug report

Modules/_elementtree.c still has four places which accept bytes. They are the result of improper translation of the PyString C API used in Python 2 to PyBytes (72b710a). The Python implementation of xml.etree.ElementTree accepts only str there, and gh-72424 declared bytes unsupported.

1. checkpath() checks a bytes path for XPath characters, so find(), findall() and findtext() with a bytes path return None/[]/'' instead of raising TypeError like iterfind() and the Python implementation.
2. Element.iter() treats b'*' as a wildcard.
3. XMLParser._setevents() accepts bytes event names, like b'start'. The Python implementation raises ValueError; the events were documented as strings in cElementTree iterparse requires events as bytes; ElementTree uses strings #53503.
4. treebuilder_handle_data() concatenates a one-byte bytes to a bytes. It is dead code: the parser always passes str.

Linked PRs

• gh-157366: Remove leftover bytes support from _elementtree #157367