Support NumPy 2 and test NumPy compatibility

[Leopold80]

Summary

• remove PyFlyt's numpy<2.0.0 upper bound
• exercise both NumPy 1.x and NumPy 2.x in the Linux test matrix
• install dependencies through the selected Python interpreter and run pip check
• update the official checkout and setup-python actions used by the workflow

Why

PyFlyt's package metadata currently prevents installation with NumPy 2 even though the existing implementation and test suite work with the latest Numba-compatible NumPy 2 release.

Validation

• Python 3.12 + NumPy 1.26.4: 123 non-GUI tests passed
• Python 3.12 + NumPy 2.4.6 + Numba 0.66.0: 123 non-GUI tests passed
• wheel build and pip check passed in both dependency environments
• macOS Apple Silicon PyBullet render smoke: human and rgb_array both passed
• Stable-Baselines3 PPO 64-step smoke run passed with NumPy 2.4.6

The GitHub workflow continues to run the complete suite under Linux/Xvfb, including rendering-related tests.