todo when min support moves to python 3.9

[maread99]

• ExchangeCalendar class methods valid_sides and default_side to be changed to class properties (like this). EDIT 06/06/22 - NOTE: seems that defining class properties in this way may not be an option from 3.11 (same reference). If it's not going to stable then leave as is.
• Remove following ExchangeCalendar methods deprecated in 4.0 (will be about time to remove them).
  • sessions_opens
  • sessions_closes
• Check if it's still necessary to have the 'near-empty' setup.py file to allow for editable installs when defining meta in pyproject.toml (reference). Remove setup.py if it's no longer needed, otherwise roll this item forwards to review when bumping min support to 3.10.
• Drop pytz in favor of zoneinfo standard library (either now or later). Looks likely that pandas 2.0 will switch to zoneinfo as its default timezone package. If it looks like pandas 2.0 could be released any time soon we might want to address this now. If it looks still to be a way off then we could roll this item forwards (to dedicated issue), although in any event pandas already supports zoneinfo and we'd be better off depending on a stdlib library than a third-party one. See Drop pytz in favor of zoneinfo standard library #322.

On bumping min version to 3.9, roll anything not addressed to a new issue 'TODO when min support moves to python 3.10'.