Update cairo_backend.py

If cairo  module import error use cairocffi

gerber/render/cairo_backend.py

@@ -17,7 +17,10 @@
 
 from .render import GerberContext
 
-import cairo
+try:
+    import cairo
+except ImportError:
+    import cairocffi as cairo
 
 from operator import mul
 import math
@@ -233,4 +236,4 @@ class GerberCairoContext(GerberContext):
         self.surface.finish()
         self.surface_buffer.flush()
         return self.surface_buffer.read()
-    
+