[ENH] switch to register_dataframe_groupby_method

[tomjemmett]

PR Description

This PR resolves #1534 by switching from register_groupby_method to register_dataframe_groupby_method and register_series_groupby_method, as the former has been removed in pandas-flavor 0.8.1.

PR Checklist

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Relevant Reviewers

• @ericmjl

[tomjemmett]

I've locally tested this in my own codebase, and it seems to work. Running tests locally I got a fair few errors, but I was getting the same errors before making the changes.

[Copilot]

Pull request overview

This PR updates pyjanitor to be compatible with pandas-flavor 0.8.1 by replacing the deprecated register_groupby_method decorator with the new specific decorators register_dataframe_groupby_method and register_series_groupby_method.

• Migrates from deprecated register_groupby_method to type-specific decorators
• Updates four functions across three files (summarise, mutate, ungroup, select)
• Adds appropriate decorators based on whether functions work with DataFrameGroupBy, SeriesGroupBy, or both

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
janitor/functions/summarise.py	Replaced register_groupby_method with register_dataframe_groupby_method for the summarise function
janitor/functions/select.py	Replaced register_groupby_method with both register_dataframe_groupby_method and register_series_groupby_method for the select function which supports both groupby types
janitor/functions/mutate.py	Replaced register_groupby_method with register_dataframe_groupby_method for both ungroup and mutate functions
CHANGELOG.md	Added entry documenting this enhancement
AUTHORS.md	Added new contributor entry

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[samukweku]

Related: #1532