Add pyi_out option to protoc command in the setup.py

[ClemensLinnhoff]

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#820

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Add pyi_out option to protoc command in the setup.py. This ways, IDEs recognize the modules and classes and autocorrect is supported:

Take this checklist as orientation for yourself, if this PR is ready for the Change Control Board:

• My suggestion follows the style and contributors guidelines.
• I have taken care about the message documentation and the fields and enums documentation.
• I have done the DCO signoff.
• My changes generate no errors when passing CI tests.
• I have successfully implemented and tested my fix/feature locally.
• Appropriate reviewer(s) are assigned.

[pmai]

CCB 2024-06-10: Should be merged as soon as development on 3.7.1 starts. Inclusion into release 3.7.0 is deemed slightly too dangerous this close to release time, with potentially undetected bugs (even though fairly unlikely), want to make sure first-time PyPI release of 3.7.0 does not contain any breaking issues.