[MAINT] Update BrainStat Version

[AliceJoubert]

Closes #1332

This PR tackles

• Updating dependencies. Note :
  Upgrading BrainStat implies to downgrade on NetNeurotools :

 - Downgrading netneurotools (0.3.0 -> 0.2.5)
 - Updating brainstat (0.4.2 -> 0.5.2)

• Drop python 3.9, adapt to python 3.13 (replacement for [MAINT] Support python 3.13 #1510)
• Removing an unused function remove_dummy_dimension
• Moving around parts of the code to follow standards

[Adam-Ismaili-92]

This PR has been tested with the following tests :

• Successful new environment creation with these new dependencies in particular :
  • Python 3.12.11
  • Nipype 1.10.0
  • Scipy 1.15.3
  • Antspyx 0.6.2
  • Brainstat 0.5.2
  • Netneurotools 0.2.5
• Successful BIDS conversion done with a subset of AIBL, using Dcm2niix 1.0.20250505
• Successful running of unit tests (apart from the usual ones linked to SPM)
• Code seems correct and respects the coding style

LGTM, Thanks for your work @AliceJoubert !