todo when min support moves to python 3.8

[maread99]

• lose memoize.py \ @lazyval in favour of @functools.cached_property
• on Answers and ExchangeCalendar classes consolidate properties returning functions decorated @functools.lru_cache with simple properties decorated with @functools.cached_property
• use Literal to annotate selection of string values, to include at least:
  • direction parameter of ExchangeCalendar, minute_to_session_label and date_to_session_label methods as Literal["next", "previous", "none"]
  • side parameter of ExchangeCalendar, test_exchange_calendar.py and calendar_helper.py methods as Literal["left", "right", "both", "neither"]
  • column parameter of Answers._get_sessions_with_times_different_to_a_contiguous_session as Literal["opens", "closes", "break_starts", "break_ends"]

[maread99]

Incorporated in #187 for v4.