DEP: Bump min version of dateutil to 2.7.3
- [x] xref https://github.com/pandas-dev/pandas/pull/32465#discussion_r404449469
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

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Not sure where to put the note in the "whatsnew"