Fix compilation with pycairo 1.17.x

Do not redefine a thing which was always defined in the included header,
since that breaks when upstream changes how they do things.

This is a holdover from the initial python2 version, since pycairo
dropped the use of downstream definition of Pycairo_CAPI during the
python3 port:

for the upstream change that broke this misuse.
1 job for master in 43 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #72647