Remove caret requirement on main dependencies

Similarly to Numpy 2.x support (!60), all the main dependencies should not have a caret requirement (^x.y.z) but a looser requirement on a working minor version (>=x.y). This create conflicts on the long term if a user need an explicit package with a major version larger than the one used in the planetary-coverage.

This applies to:

• Matplotlib
• Pillow
• Spiceypy