Add Laos holidays

[PPsyrius]

Proposed change

Add Laos holidays (en_US, lo, th localization; BANK, PUBLIC, SCHOOL, WORKDAY category).

Based on:

• https://en.wikipedia.org/wiki/Public_holidays_in_Laos
• ດຳລັດວ່າດ້ວຍ ວັນພັກ ເລກທີ 386 /ລບ (15.12.2017)

Checked with:

• https://asean.org/wp-content/uploads/2021/12/ASEAN-National-Holidays-2022.pdf
• https://asean.org/wp-content/uploads/2022/12/ASEAN-Public-Holidays-2023.pdf
• https://www.timeanddate.com/holidays/laos/
• https://www.bcel.com.la/bcel/bcel-calendar.html?y=2022
• https://www.bcel.com.la/bcel/bcel-calendar.html?year=2023
• http://www.lsx.com.la/cal/getStockCalendar.do?lang=lo (from 2011 onwards)

Closes #1215.

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency update (version deprecation/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new python-holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've added references to all holidays information sources used in this PR
• This PR is filed against beta branch of the repository
• This PR doesn't contain any merge conflicts and has clean commit history
• The code style looks good: make pre-commit command generates no changes
• All tests pass locally: make test, make tox (we strongly encourage adding tests to your code)
• The related documentation has been added/updated (check off the box for free if no updates is required)

[coveralls]

Pull Request Test Coverage Report for Build 6354322299

• 173 of 173 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 6341209667:	0.0%
Covered Lines:	9541
Relevant Lines:	9541

---

💛 - Coveralls

[KJhellico]

@PPsyrius, thank you for another great work! 👍 Please look at few suggestions.

[arkid15r]

This is so cool! I can't appreciate enough your help with Thai based calendar holidays!

I hope we still have time to include this new country calendar into v0.34.

Please take a look at my comments when you have a chance:

[PPsyrius]

Turns out my commit that fixes up the Laotian language source accidentally disabled the auto-merge...

In any case, that change is in now @arkid15r

[arkid15r]

Turns out my commit that fixes up the Laotian language source accidentally disabled the auto-merge...

That's fine, I still don't know all the features of this workflow as we've enabled it just a week or two ago.

In any case, that change is in now @arkid15r

You still have another on L49.

Otherwise LGTM, very well done!

[arkid15r]

🇱🇦 LGTM