set_page_background: raise a clear error for an invalid color - #2101

set_page_background() stored any tuple, and an invalid color only failed on the next add_page(), far from the call that set it:

• (300, 0, 0) -> ValueError: color component 300 not in range [0, 255] from add_page()
• (10, 20) -> ValueError: color component -1 not in range [0, 255], a value that was never passed
• (10, 20, 30, 40) -> TypeError: FPDF.set_fill_color() takes from 2 to 4 positional arguments but 5 were given

A color tuple is now checked when it is set, like set_fill_color() checks its components: 1 (grey level) or 3 values, each in [0, 255]. The previous background is kept when the new one is invalid.

• A unit test is covering the code added / modified by this PR
• A mention of the change is present in CHANGELOG.md
• This PR is ready to be merged