Replace: np.string_ -> np.byte_

[JerryChen97]

Context:

The usage of numpy.string_ caused quite some trouble for users with NumPy 2.x
Hopefully a direct replacement with byte_ can help a little bit.

[google-cla]

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[mhucka]

@JerryChen97 Thank you for this. I'd like to merge the changes, but before that can be done, we need you to sign off on the contributor's license agreement. Could you please check this page and follow the instructions? https://github.com/quantumlib/OpenFermion/pull/1010/checks?check_run_id=38741050105

[JerryChen97]

@mhucka Thanks a lot for reminder! I'm right now employee of another company which also features SSO therefore I'm not sure if it is proper for me to directly sign it. But I can sign this on behalf of myself during my after-work time.

[JerryChen97]

For the CI failures I'll suggest a replacement byte_ -> byte

[mhucka]

Thank you for your efforts. I've worked on NumPy 2 compatibility and this PR is subsumed by new PR #1057. Consequently, I'm going to close this one.