[rr-]

Move every import on main to the top level of its module, so that git grep -nE '^\s+(import |from \S+ import )' -- '*.py' finds only the example inside the docstring_parser.sections docstring.

• sections.default_sections imported the four style modules inside the function, because every style imports SectionSpec and the rest from sections. The shared pieces (Layout, SectionSpec, ParserCache, apply) move down to a private docstring_parser._sections, which the styles and parser import. The public docstring_parser.sections keeps its docstring, its __all__ and default_sections, and imports the styles at the top level. Its public names are unchanged.
• test_from_module_attribute_docstrings imported its own module inside the test. It now takes it from sys.modules[__name__].
• tests/_pydoctor.py is deleted rather than changed. Its try: from pydoctor.model import ... guarded an optional dependency, and nothing has loaded the file since Update CI #93: the API docs hide docstring_parser.tests with --privacy instead of --system-class.
• PLC0415 needs no config change. The PL selection already enables it in the ruff that just lint runs (v0.17.3 via prek). Only the noqa comments silenced it before, and they are gone. I checked that a nested import makes lint fail.
• No CHANGELOG line, since nothing users can see changes.
• Gates: just test passes (6612), just lint passes, just update-cases changes nothing, and parity against origin/main shows no differences. Bench against origin/main: max 1.04 on 3.14 and 1.02 on 3.8.