[`pandas-vet`] series constant series (#5802)
## Summary
Implementation for https://github.com/astral-sh/ruff/issues/5588
Q1: are there any additional semantic helpers that could be used to
guard this rule? Which existing rules should be similar in that respect?
Can we at least check if `pandas` is imported (any pointers welcome)?
Currently, the rule flags:
```python
data = {"a": "b"}
data.nunique() == 1
```
Q2: Any pointers on naming of the rule and selection of the code? It was
proposed, but not replied to/implemented in the upstream. `pandas` did
accept a PR to update their cookbook to reflect this rule though.
## Test Plan
TODO:
- [X] Checking for ecosystem CI results
- [x] Test on selected [real-world
cases](https://github.com/search?q=%22nunique%28%29+%3D%3D+1%22+language%3APython+&type=code)
- [x] https://github.com/sdv-dev/SDMetrics
- [x] https://github.com/google-research/robustness_metrics
- [x] https://github.com/soft-matter/trackpy
- [x] https://github.com/microsoft/FLAML/
- [ ] Add guarded test cases
