[Python] Fix read_comparable_areas years available presentation

[vss-2]

Bug: Python implementation of list_geobr() is missing commas.

Cause: After solving #348 merging with #349, the field years available of read_comparable_areas() is not comma separated anymore. This is happening specifically on this function because Pandas implementation of read_html() is based on CSV (comma-separated-values) parsing, and given that read_comparables_areas() is the only function with no spaces after commas, Pandas treats like a single value.

[vss-2]

Solution 1: I expected that using parse_data=True (see docs here) parameter in read_html() would solve, it didn't because of mixing both formats "YYYY, YYYY" and "YYYY,YYYY". I would recommend to format this function on README.md as a possible solution.
Solution 2: Using parameter thousands as '.' (the default is ',') solved the bug, but it doesn't seems good to rely the code on a coincidence: