support allOf in parameters schema

[romantolkachyov]

I use FastAPI to generate openapi spec. It produces allOf schemas for enums in parameters which is valid according to OpenAPI specification.

This PR fixes issue for me and removes warning (actually exception).

make html output without this fix:

Traceback (most recent call last):
  File "~/.venv/lib/python3.10/site-packages/sphinx/cmd/build.py", line 337, in build_main
    app.build(args.force_all, args.filenames)
  File "~/.venv/lib/python3.10/site-packages/sphinx/application.py", line 378, in build
    self.builder.build_update()
  File "~/.venv/lib/python3.10/site-packages/sphinx/builders/__init__.py", line 297, in build_update
    self.build(to_build,
  File "~/.venv/lib/python3.10/site-packages/sphinx/builders/__init__.py", line 318, in build
    updated_docnames = set(self.read())
  File "~/.venv/lib/python3.10/site-packages/sphinx/builders/__init__.py", line 425, in read
    self._read_serial(docnames)
  File "~/.venv/lib/python3.10/site-packages/sphinx/builders/__init__.py", line 477, in _read_serial
    self.read_doc(docname)
  File "~/.venv/lib/python3.10/site-packages/sphinx/builders/__init__.py", line 536, in read_doc
    publisher.publish()
  File "~/.venv/lib/python3.10/site-packages/docutils/core.py", line 234, in publish
    self.document = self.reader.read(self.source, self.parser,
  File "~/.venv/lib/python3.10/site-packages/sphinx/io.py", line 106, in read
    self.parse()
  File "~/.venv/lib/python3.10/site-packages/docutils/readers/__init__.py", line 76, in parse
    self.parser.parse(self.input, document)
  File "~/.venv/lib/python3.10/site-packages/sphinx/parsers.py", line 83, in parse
    self.statemachine.run(inputlines, document, inliner=self.inliner)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 169, in run
    results = StateMachineWS.run(self, input_lines, input_offset,
  File "~/.venv/lib/python3.10/site-packages/docutils/statemachine.py", line 233, in run
    context, next_state, result = self.check_line(
  File "~/.venv/lib/python3.10/site-packages/docutils/statemachine.py", line 445, in check_line
    return method(match, context, next_state)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 2790, in underline
    self.section(title, source, style, lineno - 1, messages)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 325, in section
    self.new_subsection(title, lineno, messages)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 391, in new_subsection
    newabsoffset = self.nested_parse(
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 279, in nested_parse
    state_machine.run(block, input_offset, memo=self.memo,
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 195, in run
    results = StateMachineWS.run(self, input_lines, input_offset)
  File "~/.venv/lib/python3.10/site-packages/docutils/statemachine.py", line 233, in run
    context, next_state, result = self.check_line(
  File "~/.venv/lib/python3.10/site-packages/docutils/statemachine.py", line 445, in check_line
    return method(match, context, next_state)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 2357, in explicit_markup
    nodelist, blank_finish = self.explicit_construct(match)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 2369, in explicit_construct
    return method(self, expmatch)
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 2106, in directive
    return self.run_directive(
  File "~/.venv/lib/python3.10/site-packages/docutils/parsers/rst/states.py", line 2156, in run_directive
    result = directive_instance.run()
  File "~/.venv/lib/python3.10/site-packages/sphinxcontrib/openapi/directive.py", line 56, in run
    return renderer_cls(self.state, self.options).render(spec)
  File "~/.venv/lib/python3.10/site-packages/sphinxcontrib/openapi/renderers/abc.py", line 40, in render
    for line in self.render_restructuredtext_markup(spec):
  File "~/.venv/lib/python3.10/site-packages/sphinxcontrib/openapi/renderers/_httpdomain_old.py", line 55, in render_restructuredtext_markup
    yield from openapihttpdomain(spec, **self._options)
  File "~/.venv/lib/python3.10/site-packages/sphinxcontrib/openapi/openapi31.py", line 322, in _httpresource
    type_ = _get_type_from_schema(param["schema"])
  File "~/.venv/lib/python3.10/site-packages/sphinxcontrib/openapi/openapi31.py", line 305, in _get_type_from_schema
    for t in schema["anyOf"]:
KeyError: 'anyOf'

Exception occurred:
  File "~/.venv/lib/python3.10/site-packages/sphinxcontrib/openapi/openapi31.py", line 305, in _get_type_from_schema
    for t in schema["anyOf"]:
KeyError: 'anyOf'

Schema example:

---
openapi: "3.1.0"
info:
  title: "Reproducer for issue #163"
  version: 2.0.0
paths:
  /users:
    get:
      summary: Get all users.
      parameters:
        - in: query
          name: role
          schema:
            # This is how FastAPI renders Enum in parameters
            allOf:
              - type: "string"
                enum: ["admin", "member", "reader"]
                title: Example Enum param
            default: "admin"
      responses:
        "200":
          description: A list of all users.
          content:
            application/json:
              schema:
                type: object
                properties:
                  success:
                    type: bool