Breaking changes

Multiple builds available, with the alternative build enhancing multiple XML subclassing scenario. See relevant README section for detail. Thanks to @scanny for the driving force behind #51.
Mypy 1.11 required, which introduced backward incompatible @typing.overload changes.
lxml.html.clean stub depreated, lxml 5.2.0 completely removes the submodule due to multiple security issues. Corresponding code and type definitions are split into a new independent repo.

Features

(#56) Replace typing.TypeGuard with typing.TypeIs
Use callback protocol for more precise element and ElementMaker factory function typing
lxml.etree.ICONV_COMPILED_VERSION exported since 5.2.2
Special handling for ObjectifiedElement and HTMLElement in lxml.cssselect.CSSSelector and various cssselect() methods
html.builder shorthands return more precise element type for certain HTML elements. For example, html.builder.LABEL(), corresponding to <LABEL> tag, yields LabelElement.
More precise etree.Extension() annotation depending on supplied namespace
Stricter namespace argument type in _Element ElementPath methods
For lxml.builder.ElementMaker class:
- Provide better hint in __call__() argument
- Accepts namespace tuple in nsmap argument
- Export private properties
For lxml.sax module:
- Export private properties in various classes
- Explicitly list all inherited methods in ElementTreeContentHandler class, as method arguments names are different from superclass ones
Alert etree.HTMLParser users to remove deprecated strip_cdata argument

Some _Element related input arguments fixed to use typing.Sequence instead of Interable, as _Element is already an Iterable itself. Supplying _Element where a proper Iterable is expected would cause problem.
Similar situation arises for str or byte in tag selector argument; use typing.Collection to alert user more clearly.
None can't be used as etree.strip_*() argument
Some etree.DocInfo read-only properties can't be None
Fix etree.Resolver method return types
Avoid exception raising arg combinations in html.html5parser.HTMLParser

The usual static stub to runtime test migration:
- Part of basic _Element tests and its find*() methods
- More extensive _Attrib tests
Use ruff to replace black and isort as code formatter
Migrate stub tests to support pytest-mypy-plugins ⩾ 2.0
Use pdm-backend as build backend due to its more versatile versioning support