Address `pandas.value_counts()` deprecation warnings

[dlqqq]

Description

The build-changelog step now emits several warnings to the terminal:

 /opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/site-packages/github_activity/github_activity.py:406: FutureWarning: pandas.value_counts is deprecated and will be removed in a future version. Use pd.Series(obj).value_counts() instead.
  item_commentors_counts = pd.value_counts(item_commentors)
/opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/site-packages/github_activity/github_activity.py:406: FutureWarning: value_counts with argument that is not not a Series, Index, ExtensionArray, or np.ndarray is deprecated and will raise in a future version.
  item_commentors_counts = pd.value_counts(item_commentors)
/opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/site-packages/github_activity/github_activity.py:406: FutureWarning: pandas.value_counts is deprecated and will be removed in a future version. Use pd.Series(obj).value_counts() instead.
  item_commentors_counts = pd.value_counts(item_commentors)
/opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/site-packages/github_activity/github_activity.py:406: FutureWarning: value_counts with argument that is not not a Series, Index, ExtensionArray, or np.ndarray is deprecated and will raise in a future version.
  item_commentors_counts = pd.value_counts(item_commentors)
/opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/site-packages/github_activity/github_activity.py:413: FutureWarning: pandas.value_counts is deprecated and will be removed in a future version. Use pd.Series(obj).value_counts() instead.
  comment_contributor_counts = pd.value_counts(comment_helpers)
/opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/site-packages/github_activity/github_activity.py:413: FutureWarning: value_counts with argument that is not not a Series, Index, ExtensionArray, or np.ndarray is deprecated and will raise in a future version.
  comment_contributor_counts = pd.value_counts(comment_helpers)

Reproduce

See https://github.com/jupyterlab/jupyter-chat/actions/runs/12487228881/job/34848157691.

Expected behavior

We should follow their recommendation to suppress these warnings and prevent Jupyter Releaser from breaking in the future.

[jtpio]

Thanks @dlqqq for the report 👍

This seems to be coming from github-activity, which jupyter-releaser uses to generate the changelog.

And tracked in this issue: executablebooks/github-activity#91