feat: Pydantic v2 + async/internal cleanup (v1.2.0)

[svalench]

Summary

This PR modernizes fastapi_viewsets for Pydantic v2 and removes
several long-standing rough edges around async support, configuration
loading and code duplication. There are no breaking changes for
applications that already pass Pydantic v2 schemas.

Pydantic v2 first

• CRUD handlers now serialize request bodies via
  model_dump(exclude_unset=...) instead of the legacy dict(item)
  call.
• PATCH semantics fixed. With Pydantic v2,
  model_dump(exclude_unset=True) only returns fields the client
  actually sent. BaseViewset.update_element and
  AsyncBaseViewset.update_element use this for partial=True, so
  unset fields are no longer accidentally overwritten with Pydantic
  defaults. This was a real correctness bug in 1.1.0.
• New compatibility helper fastapi_viewsets._compat.model_to_dict
  transparently supports both Pydantic v1 (.dict) and v2
  (.model_dump) models.
• tests/conftest.py migrated to
  model_config = ConfigDict(from_attributes=True).
• pydantic>=2.5,<3 is now a hard runtime dependency.

Async / configuration cleanup

• db_conf.py resolves SQLAlchemy globals lazily via module-level
  __getattr__. Importing fastapi_viewsets no longer creates engines
  or imports SQLAlchemy when another ORM is selected, and no longer
  fails when an async driver such as aiosqlite is missing.
• The sync→async URL conversion (sqlite:/// →
  sqlite+aiosqlite:///, etc.) lives in a single helper,
  to_async_database_url. Previously it was duplicated across
  db_conf, factory and the SQLAlchemy adapter.
• Removed the duplicate _default_adapter singleton from db_conf;
  ORMFactory.get_default_adapter() is now the only source of truth.
  ORMFactory.reset_default_adapter() was added for tests and hot
  reloads.

Internal refactor

• BaseViewset.register and AsyncBaseViewset.register were two
  near-identical ~90-line methods. They are now a single
  _RegisterMixin.register shared by both classes.
• Replaced bare except: pass blocks around annotation patching with a
  narrow (AttributeError, TypeError) guard.
• Renamed the no-op auth dependency from butle to
  _noop_dependency; butle is kept as a backward-compatible alias.
• Trimmed unused imports (os, duplicate Any).

Packaging

• Added a PEP 621 pyproject.toml (with setuptools build backend).
  setup.py is now a one-line shim.
• Bumped python_requires to >=3.9 to match the CI test matrix
  and Pydantic v2 requirements.
• Added [project.optional-dependencies] lint (ruff + black + mypy)
  and pre-configured [tool.ruff], [tool.black], [tool.mypy].
• FastAPI minimum bumped to >=0.110 to align with Pydantic v2 support.

Tests

• 237 / 245 tests pass locally on CPython 3.12 — no regressions
  vs. baseline. The 8 remaining failures are pre-existing
  Tortoise/Peewee issues caused by upstream library changes
  (e.g. tortoise.exceptions.ConfigurationError: AppConfig.models must be a non-empty list of strings) and are not introduced by this PR.
• Coverage: 83% (above the 70% gate).

Backward compatibility

• The butle dependency name still works.
• Pydantic v1 schemas keep working via the model_to_dict shim.
• BaseViewset / AsyncBaseViewset public APIs are unchanged.

Files

• New: fastapi_viewsets/_compat.py,
  fastapi_viewsets/_register.py, pyproject.toml,
  RELEASE_1.2.0.md.
• Modified: fastapi_viewsets/__init__.py,
  fastapi_viewsets/async_base.py, fastapi_viewsets/db_conf.py,
  fastapi_viewsets/orm/factory.py,
  fastapi_viewsets/orm/sqlalchemy_adapter.py, setup.py,
  tests/conftest.py, README.md, RELEASE_NOTES.md.