BUG: Runtime warning with groupby/tail when `None` appears in group column

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• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas
df = pandas.DataFrame(
    [
        ["a", 1],
        ["a", 2],
        [None, 0],
        ["b", 2],
        ["b", 3],

    ],
    columns=["x", "y"],
)
df.groupby("x", dropna=False).tail(1)   # Succeeds
df.groupby("x", dropna=True).tail(1)   # Produces warning

Issue Description

👋 In pandas main, a groupby/tail on a DataFrame which contains nulls in the grouped column produces a RuntimeWarning suggesting a mishandled case in indexing logic:

/lib/python3.8/site-packages/pandas/core/groupby/indexing.py:217: RuntimeWarning: invalid value encountered in remainder
  mask &= offset_array % step == 0

If I set dropna=False, the snippet succeeds without producing a warning.

Based on the warning location and git blame, it may be related to https://github.com/pandas-dev/pandas/pull/42947

Expected Behavior

The above groupby should succeed without producing a warning.

Installed Versions

Pandas main, as well as 1.4.

[mroeschke]

Closing as it seems this was a numpy isssue

[ian-r-rose]

Thanks @rhshadrach! Upgrading numpy is a fine workaround from my perspective.