Add Guyana holidays

[arkid15r]

Links:

• https://en.wikipedia.org/wiki/Public_holidays_in_Guyana
• https://www.timeanddate.com/holidays/guyana/
• https://moha.gov.gy/public-holidays/

[bors-ltd]

Looks like before we integrate Guyana, we'll have to fix French Guiana using their code GY, instead of the expected GF.

I'm willing to contribute, but I guess it's not just a matter of search and replace. We'll have to go through a deprecation period.

The PR #1063 is fixing this, but it seems stalled.

[arkid15r]

I think you can just add a deprecation warning and make GF available. We don't need to wait for GY removal from FR holidays as it won't clash with the supported ISO-3166 entities.

The PR #1063 is fixing this, but it seems stalled.

Oh yeah, that's an old one. We didn't have a good understanding of a direction for holidays at that time yet. I think we can close that one.

@KJhellico any comments/suggestions?

[KJhellico]

Yes, I think we can postpone the addition of overseas territories, focusing on the countries from UN list.