CLN: Update files (as per #36450) to Python 3.7+ syntax
Ran `pyupgrade` on all files unchecked in #36450. formatter pre-commit 
hook had no complaints; removed unnecessary imports from the following
to satisfy `flake8` hook:

```
pandas/tests/io/test_sql.py
pandas/test/series/test_io.py
```

- [x] closes #36450
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

thanks @ahgamut very nice!