meson: Make the `pycairo` option a feature

So user have more control on whether to build it or not
11 jobs for pycairo_feature in 10 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Build And Test
passed #750898
flatpak
gnome-master

00:02:20

failed #750899
flatpak allowed to fail
gnome-master-gtk4

00:01:53

passed #750896
pypy3.6

00:05:02

passed #750892
python3.5

00:08:34

passed #750893
python3.6

00:04:08

passed #750894
python3.7

00:03:58

passed #750895
python3.8

00:05:52

passed #750890
win32-ps
python3-mingw32

00:02:59

passed #750891
win32-ps
python3-mingw64

00:03:30

passed #750897
xenial-i386-py3

00:02:04

 
  Coverage
passed #750900
coverage

00:01:24

 
Name Stage Failure
failed
gnome-master-gtk4 Build And Test
    return _bootstrap._gcd_import(name[level:], package, level)
gi/overrides/Gtk.py:168: in <module>
class Container(Gtk.Container, Widget):
gi/module.py:123: in __getattr__
raise AttributeError("%r object has no attribute %r" % (
E AttributeError: 'gi.repository.Gtk' object has no attribute 'Container'
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1