Add 2020 data

[chekos]

🚀 Feature Request

The census did not release 2020 1-year ACS data because of COVID but they have it under the experimental section (with experimental weights). There is a 5-year PUMS dataset though.

📎 Additional context

Add the experimental flag.

[chekos]

This has been addressed by the recent refactoring work (Phases 0-4). 2020 data is now fully supported across all Census data products: get_decennial(year=2020), get_acs(year=2020), get_pums(year=2020), get_estimates(vintage=2020), with appropriate handling for the 2020 ACS PUMS 5-Year-only limitation due to COVID-19.