[python-package] prefix several internal functions with _

[Madnex]

Contributes to #5313, by prefixing the following internal functions in lightgbm.basic with _:

• cfloat32_array_to_numpy()
• cfloat64_array_to_numpy()
• cint32_array_to_numpy()
• cint64_array_to_numpy()
• c_str()
• is_1d_list()
• list_to_1d_numpy()
• cfloat32_array_to_numpy()

[Madnex]

@microsoft-github-policy-service agree

[jameslamb]

Thanks very much for your help with this! Please see the one suggestion I made.

I also made two changes to this PR:

• listed the specific changes object names in the description (since the PR title saying "multiple" isn't enough to indicate which objects you intended to change)
• made the PR title slightly more specific so it will be a bit easier for people reading the release notes to understand what this PR did

Also, I noticed that you did one commit per object. That's totally fine if that helped you to organize your thoughts, but if you worked that way for the benefit of LightGBM, I want to let you know you don't need to do that extra work here in the future. This project uses squash merging, which means that all of the changes from a PR are combined into a single commit on master when the PR is merged.

[Madnex]

Thanks for the hints @jameslamb! I will arrange the future changes for this issue than in less commits and mention the changed definitions explicitly.

[StrikerRUS]

LGTM, thanks!

[jameslamb]

Thanks very much! We'll merge this once we fix the project's CI issues (unrelated to your PR).

[jameslamb]

@Madnex sorry for the delay! We've now fixed the project's CI issues.

Sorry for the inconvenience, but could you please merge the latest changes from master into that branch? Once you do that and CI runs successfully, I'd be happy to merge this.

Thanks so much for your continued help with LightGBM!

[github-actions]

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