Fixes: #47

Fixes #38

Fixes #15

This commit centralizes the definitions of formatter lists used for
testing and resolves an ImportError that was occurring in PyPy
environments during test collection.

Key changes:

1.  Centralized Formatter Lists in `tests/__init__.py`:
    *   Added `from __future__ import annotations` for modern type hints.
    *   Defined `ALL_FORMATTERS: List[Type[BaseJsonFormatter]]` containing
        actual formatter class objects (`JsonFormatter`, and conditionally
        `OrjsonFormatter`, `MsgspecFormatter` based on availability of
        their underlying packages).
    *   Defined `ALL_FORMATTER_PATHS: List[str]` derived from
        `ALL_FORMATTERS` (e.g., `["pythonjsonlogger.json.JsonFormatter", ...]`).
    *   The conditional inclusion of formatters now correctly relies on
        `pythonjsonlogger.ORJSON_AVAILABLE` and
        `pythonjsonlogger.MSGSPEC_AVAILABLE` flags to prevent import
        attempts if dependencies are missing.

2.  Updated `tests/test_formatters.py`:
    *   Removed its local definition of `ALL_FORMATTERS`.
    *   Now imports and uses the centralized `ALL_FORMATTERS` list from
        `tests.__init__`.

3.  Updated `tests/test_dictconfig.py`:
    *   Removed its local definition of `SUPPORTED_DICTCONFIG_FORMATTERS`.
    *   Removed direct top-level imports of `pythonjsonlogger.orjson` and
        `pythonjsonlogger.msgspec` modules, which were causing
        `MissingPackageError` on PyPy environments where these optional
        dependencies are not installed. The `test_dictconfig.py` now relies
        on `ALL_FORMATTER_PATHS` imported from `tests.__init__`, where the
        conditional logic correctly handles availability.
    *   The parameterized test
        `test_external_reference_support_parameterized` now correctly uses
        the imported `ALL_FORMATTER_PATHS`.

This refactoring ensures a single source of truth for formatter lists,
improves maintainability, and crucially fixes the test collection errors
on PyPy environments. All linters and tests now pass across all
configured environments in `tox`.

This commit refines `tests/test_dictconfig.py` to concentrate on
testing aspects unique to `python-json-logger`'s interaction with
`logging.config.dictConfig`, rather than general `dictConfig` behaviors
that are the responsibility of the standard library.

Key changes:

1.  Removed Redundant Tests:
    - `test_disable_existing_loggers`: This test primarily verified the
      standard `disable_existing_loggers` flag of `dictConfig`.
    - `test_multiple_handlers_and_formatters`: While it used the library's
      formatter, the core functionality tested (routing to multiple
      handlers) is standard `dictConfig` behavior.
    - `test_filter_configuration`: Testing filter application is also
      a standard `dictConfig` feature.
    - Associated helper classes (`TestStream1`, `TestStream2`, `LevelFilter`)
      and global instances (`test_stream_1_instance`, etc.) were removed
      as they were only used by these tests.

2.  Retained Focused Tests:
    - `test_external_reference_support_parameterized`: Kept as it tests
      the crucial `ext://` reference resolution for formatter options,
      which is a key integration point for `python-json-logger` with
      `dictConfig`. This test is also parameterized to run against all
      supported formatter types (`JsonFormatter`, `OrjsonFormatter`,
      `MsgspecFormatter` via `ALL_FORMATTER_PATHS`).
    - `test_json_encoder_option` and `test_json_serializer_option`: Kept
      as they specifically test `ext://` references for configuring custom
      encoders and serializers, a `dictConfig`-specific interaction for
      the library's formatters.

This streamlining ensures that `test_dictconfig.py` is more focused on
validating the unique aspects of `python-json-logger`'s support for and
integration with `dictConfig`, reducing redundancy with tests that would
be more appropriate for the standard library's own test suite.

All linters and the full `tox` test suite pass with these changes.