Flasktester

- Improve comments.
- Activate _Python 3.13_ and _Pypy 3.10_ in GitHub CI.
- Restrict CI to _main_ branch.
- Add explicit `bcrypt` dependency for tests.
- Allow mixing `json` and `data` parameters by merging into `data`.

Fix stupid bug introduced in v4.1.

Add support for `pydantic` and `dataclass` parameters with `data` and `json`.

- improved documentation
- improved tests
- remove deprecated `check` method and `FLASK_TESTER_URL` env
- simplify code

- Only use `FLASK_TESTER_APP`, hide `FLASK_TESTER_URL`, which is only kept for upward compatibility and is deprecated.
- Improved documentation, including a working `app2`.

- Improve failure behavior and testing.