bugfix for numpy 2.0

[Liuqi2002519]

Description of proposed changes

• remove all occurrences of np.string_ with np.bytes_, as the former has been removed in numpy 2.0

• utils.time_func.get_design_matrix4polynomial_func(): swtich from np.math.factorial() to math.factorial(), as the former is removed from numpy 2.0

Reminders

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [ ] Pass Circle CI test (green)
• [x] Pass local test
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.

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