v2.8.0: Fix for `MaskablePPO` and markdown doc

Breaking Changes:

• Removed support for Python 3.9, please upgrade to Python >= 3.10
• Upgraded to Stable-Baselines3 >= 2.8.0
• Set strict=True for every call to zip(...)

New Features:

• Added official support for Python 3.13

Bug Fixes:

• Fixed MaskablePPO and RecurrentPPO inaccurate n_updates counting when target_kl early exits the training loop
• Fixed RecurrentPPO and MaskablePPO forward and predict not reshaping the action before clipping it (@immortal-boy)
• Do not call forward() method directly in RecurrentPPO (@immortal-boy)
• Fixed MaskableCategorical.apply_masking() crashing with ValueError: Simplex when cached probs deviate from sum=1 in float32 with large action spaces (torch 2.9+) (@kirann-05)

Documentation:

• Switched to markdown documentation (using MyST parser)

New Contributors

• @immortal-boy made their first contribution in #320
• @kirann-05 made their first contribution in #326

Full Changelog: v2.7.1...v2.8.0