[cairo] Qahirah: Approximately-Equal Vectors, Matrcies & Rectangles

[Lawrence D'Oliveiro]

I’ve felt for a while that it would be useful for Qahirah
<https://github.com/ldo/qahirah>, my high-level Python binding for
Cairo, to have “approximately-equal” comparisons for Vector, Matrix and
Rect objects.

One of the new features in Python 3.5 is the math.isclose
<https://docs.python.org/3/library/math.html#math.isclose> function,
which provides exactly such a comparison for real numbers.

So I have built on this to add “iscloseto” methods to those Qahirah
objects, with the same options for specifying both relative and
absolute tolerances for the comparisons.