Feature/add nyse holidays

[nadime]

Change to add NYSE holidays as an additional "financial markets" set of holidays. These were still done within the "countries" infrastructure, but perhaps once a few more financial markets are added we should modify the structure slightly.

I've included only "full day" holidays for the NYSE historically and done my best to match with a few sources:
https://s3.amazonaws.com/armstrongeconomics-wp/2013/07/NYSE-Closings.pdf
and
https://nyseholidays.blogspot.com/2012/11/nyse-holidays-from--.html where year1/year2 are always a year ending in X1 and X0 (e.g. 1941-1950).

I've tried my best to conform to the standards I saw, for example adding " (Observed)" at the end of holidays when the observed holiday does not fall on the actual holiday, but please let me know if you see anything else you'd like changed.

Cheers!

[mborsetti]

Very nice work!

Per https://github.com/dr-prodigy/python-holidays/issues/458, the ISO 20022 code to be used here is XNYS.

@dr-prodigy not sure how you want to categorize these so they can grow (we already have something for European markets).

[coveralls]

Pull Request Test Coverage Report for Build 2116314726

• 175 of 175 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 98.942%

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Totals
Change from base Build 2102842664:	0.03%
Covered Lines:	6349
Relevant Lines:	6366

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💛 - Coveralls

[nadime]

Ah yes, of course - you're referring to the README, right (fixed there)? Or did I miss somewhere else you want the right MIC?

[nadime]

Are we waiting on anything here? Can we merge this into beta?

[dr-prodigy]

Hi @nadime thanks for your great work!
I in fact decided to apply some refactoring to allow for a better financial markets integration, adding a dedicated package (financial), adding a specific method for retrieving those classes only (list_supported_financial()) and reviewing tests accordingly.
I also added another synonym for the new class (XNYS) as suggested by @mborsetti.
Merged in beta, thx! 👍