Add French school holidays

[francoiskha]

Is your feature request related to a problem? Please describe. French public holidays are working fine but the French school holidays currently are not implemented.

Describe the solution you'd like French school holidays are divided into 3 main zones (A, B and C) for the "mainland" (aka Métropole in French) plus other divisions for overseas territories. All those overseas territories are already registered as a subdivision in france.py except Corsica (aka Corse in French).

The information source is the relevant dataset from the French government open data website (FR) The difficulty here is that the French administration publishes, each year, the school holidays calendar only 2 years in advance. There is no sure way to predict the calendar further in advance. Moreover, the calendar may change after the original publication. (French law (FR)).

The Holidays lib has a small dependency footprint that does not permit data fetching. I suppose this is intentional so my proposal is to manually fetch and implement the changes each year.

Describe alternatives you've considered None, but I am very new to this library and very inexperienced with it. Please do not hesitate to add more context or ideas.

Additional context Issue #2506 is a similar request for Belgian school holidays.

Rough implementation proposal:

1. To add the "Metropole Zone A", "Metropole Zone B", "Metropole Zone C" and "Corsica" subdivisions
2. To implement the supported_categories SCHOOL
3. To implement the _populate_subdiv_<ID>_school_holidays for each relevant subdivision.