text: raise a clear error for a NaN or infinite position - #2141

FPDF.text(x, y, ...) writes a NaN or infinite position straight into the content stream, which is not a valid PDF number:

pdf.text(float("nan"), 10, "Hello")   # writes: BT nan 813.54 Td (Hello) Tj ET

set_x(), set_y() and set_xy() already reject these with _check_coordinate() (#2121), so text() now calls the same helper for x and y, before anything is written. Finite positions, including negative ones, are unchanged.

This does not overlap with #2139, which covers line(), polyline(), polygon(), ellipse() and circle().

Added test_text_not_finite_position (NaN, inf and -inf, for x and for y); it fails on master. The full test suite passes except test_image_with_explicit_dimensions, which also fails on master in my environment