namespace consistency for test_dtype.py
- [x ] references #37838
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Can you drop the first commit?

So
```
git fetch --all --prune
git rebase -i upstream/master
```
then, in the interactive window, change the first `pick` to `drop`. Then (you may need to run this one twice)
```
pre-commit run --files pandas/tests/arithmetic/test_interval.py
```
Then,
```
git add pandas/tests/arithmetic/test_interval.py
git commit -m "fixup"  # or whatever message you like
git push origin namespace -f
```

Let me know if anything trips you up here