[fft] Optimize fftfreq logic

[Routhleck]

This pull request includes changes to the brainunit/fft/_fft_change_unit.py file, specifically focusing on improving exception handling and compatibility with different Python versions. The most important changes include replacing generic exceptions with KeyError and adding version checks for Python 3.10 and above.

Improvements to exception handling:

• Replaced generic except statements with except KeyError in the fftfreq and rfftfreq functions. [1] [2]

Compatibility with Python versions:

• Added version checks for Python 3.10 and above to handle the device parameter in the fftfreq and rfftfreq functions. [1] [2]