commit 0ab00e4991f3a94979cb476ed1c690614a7eb91a
    Author: Daniel Hahler <git@thequod.de>
    Date:   Wed Mar 25 14:37:29 2015 +0100

        pygi-foreign-cairo.c: fix include for py3cairo.h
        
        The pkg-config info includes the "pycairo" folder already:
        
            % cat ~/.pyenv/versions/3.4.3/lib/pkgconfig/py3cairo.pc
            prefix=/home/user/.pyenv/versions/3.4.3
        
            Name: Pycairo
            Description: Python 3 bindings for cairo
            Version: 1.10.1
            Requires: cairo
            Cflags: -I${prefix}/include/pycairo
            Libs:

    diff --git a/gi/pygi-foreign-cairo.c b/gi/pygi-foreign-cairo.c
    index 5937759..8e76529 100644
    --- a/gi/pygi-foreign-cairo.c
    +++ b/gi/pygi-foreign-cairo.c
    @@ -28,7 +28,7 @@
     #include <pycairo.h>
     static Pycairo_CAPI_t *Pycairo_CAPI;
     #else
    -#include <pycairo/py3cairo.h>
    +#include <py3cairo.h>
     #endif
     
     #include <cairo-gobject.h>

I am a bit surprised that this wasn't reported and fixed already.

I have not investigated, but pygi-foreign-cairo.c is probably not usually included/used?

I've came across this through a recipe to install pygtk3 in pyenv (a wrapper for multiple Python versions):
https://gist.github.com/blueyed/b4424dac5749b683c9fc