chore: prepare for 3.0.0rc4

[henryiii]

Description

Preparing for the final (?) RC.

I used this ChatGPT generated script:

import re
from collections import Counter

# Replace this with reading from your changelog file if needed
with open("docs/changelog.md", "r", encoding="utf-8") as f:
    text = f.read()

# Find all PR references like [#5705](https://github.com/pybind/pybind11/pull/5705)
pattern = r'\[#(\d+)\]\(https://github\.com/pybind/pybind11/pull/\1\)'
matches = re.findall(pattern, text)

# Count occurrences
counter = Counter(matches)

# Find duplicates
duplicates = [pr for pr, count in counter.items() if count > 1]

if duplicates:
    print("Duplicate PRs found:")
    for pr in duplicates:
        print(f"- #{pr} (appears {counter[pr]} times)")
else:
    print("No duplicate PRs found.")

To find and remove a few duplicates.

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📚 Documentation preview 📚: https://pybind11--5736.org.readthedocs.build/

[rwgk]

Thanks!