refactor: full `scikit-learn` compatibility + general refactor

[eonu]

• Use scikit-learn ≥1.4 metadata routing for full API compatibility
• Switch from setup.py to pyproject.toml
• Switch to poetry for dependency management
• Use tox for documentation building, unit tests and linting
• Add pre-commit hooks
• Add coverage checks via coveralls
• Drop support for Python <3.11
• Support Python 3.11 and 3.12
• Add issue and pull request templates
• Add license information to .py files
• Update repository documentation
• Use cliff for CHANGELOG.md generation
• Update Sphinx documenation
• Switch to numpy docstring format (via napoleon)
• Move library code from lib/ to sequentia/
• Move unit tests from lib/test to tests/
• Add sequentia.enums module for configuration values.
• Add sequentia.version sub-package for versioning information.
• Add sequentia._internal sub-package for internal logic.
• Remove sequentia.pipeline module.
• Make topologies internal
• Remove pydantic validation classes for model inputs
• Remove pydantic validation for numpy arrays
• Use key-word only arguments more widely (e.g. for lengths)
• Use pydantic.validate_call for validating constructors and other functions
• Use type hinting
• Use ABCMeta metaclass for base classes (e.g. ClassifierMixin/RegressorMixin and BaseHMM)
• Use abc decorators for abstract methods
• Avoid unnecessarily redefining methods on subclasses for only documentation purposes
• Add predict_log_proba to classifiers
• Move majority of validation logic to sequentia._internal._validation module
• Prefix all HMM parameter setter functions with set_state
• Rename set_transitions to set_state_transition_probs`
• Rename set_state_emissions to set_state_emission_probs
• Rename set_state_covariances to set_state_covars
• Use static methods to define default HMM parameters
• Handle KNN querying for single training sequences and single query sequences
• Update IndependentFunctionTransformer to match FunctionTransformer from scikit-learn ≥1.4 API
• Use Helpers fixture class for accessing helper methods in unit tests