[pygobject] docs: Add pycairo requires for development setup

[Christoph Reiter <creiter src gnome org>]

commit c85075972b7fe16d0dd05d9136fdb8c1f3e8f39d
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sat May 5 06:08:59 2018 +1000

    docs: Add pycairo requires for development setup

 docs/devguide/dev_environ.rst | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/docs/devguide/dev_environ.rst b/docs/devguide/dev_environ.rst
index 265a551f..db316784 100644
--- a/docs/devguide/dev_environ.rst
+++ b/docs/devguide/dev_environ.rst
@@ -257,6 +257,7 @@ additional steps:
     pipenv --python 3
     pipenv install pytest
     pipenv install flake8
+    pipenv install pycairo
     pipenv shell