Distinguish str from Iterable[str] in arguments

[abelcheung]

It is a common typing problem that str can actually be used in Iterable[str] arguments and most likely cause undesirable effect. We should find all places where such problem may occur (including Collection[str] and Sequence[str] as well as bytes counterparts), and use warnings.deprecated to intercept str usage. Potentially problematic functions / methods below:

• etree._ElementTree.write(method='c14n',inclusive_ns_prefixes=...) (06f868b)
• etree.C14NWriterTarget.__init__(...)
• etree.Extension(function_mapping=...)
• etree.canonicalize(...)
• etree.cleanup_namespaces(keep_ns_prefixes=...)
• etree.fromstringlist(strings=...) (bc8798b)
• etree.strip_attributes(__attrib=...)
• etree.tostring(method='c14n',inclusive_ns_prefixes=...) (06f868b)
• html.soupparser.fromstring(exclude_encodings=...) (68facbc)
• html.soupparser.parse(exclude_encodings=...) (68facbc)

Note that only pyright would provide deprecation warnings; it remains unsupported as of mypy 1.13.