Add Juneteenth holiday to NYSE calendar

[vassilios]

Effective from 2022, NYSE (and other exchanges like NASDAQ and CBOE) will be closed on Juneteenth.

Under this change to NYSE Rule 7.2, the Exchange will be closed on June 19th of each year going forward. If the holiday falls on a Saturday, the Exchange will be closed the preceding Friday – and if it falls on a Sunday, the Exchange will be closed the succeeding Monday. This rule change brings the number of NYSE market holidays to ten.

See relevant announcement:
https://www.thecorporatecounsel.net/blog/2021/10/nyse-makes-juneteenth-a-new-market-holiday.html

NYSE holidays schedule: https://www.nyse.com/markets/hours-calendars

There are other pull requests which implement Juneteenth for other calendars (see for example:
#195 )

For the NYSE calendar, we would require this to start from year 2022 using something like the following:

  6:
  - name: Juneteenth National Independence Day
    regions: [nyse]
    mday: 19
    observed: to_weekday_if_weekend(date)
    year_ranges:
      from: 2022