gnu: python-pycairo: Fix build with with python-3.4.3.

* gnu/packages/patches/pycairo-wscript.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/gtk.scm (python-pycairo)[source]: Use it.
  [build-system]: Use waf-build-system.
  [native-inputs]: Add python-waf.
  [arguments]<#:phases>: Use 'modify-phases'.  Add 'patch-waf phase.
* gnu/packages/gtk.scm (python2-pycairo): Adjust accordingly.