Revert "Revert "stack/slapos: version ↑ jsonschema 3.0.0a3""

This reverts commit c2a7ed05.

stack/slapos.cfg

@@ -262,18 +262,26 @@ erp5.util = 0.4.69
 feedparser = 5.2.1
 
 # Required by:
-# jsonschema==2.6.0
+# jsonschema==3.0.0a3
 functools32 = 3.2.3.post2
 
 # Required by:
-# jsonschema==2.6.0
+# jsonschema==3.0.0a3
+attrs = 18.2.0
+
+# Required by:
+# jsonschema==3.0.0a3
+pyrsistent = 0.14.5
+
+# Required by:
+# jsonschema==3.0.2
 setuptools-scm = 3.5.0
 
 ipaddress = 1.0.23
 
 # Required by:
 # slapos.cookbook==1.0.143
-jsonschema = 2.6.0
+jsonschema = 3.0.2
 
 # Required by:
 # slapos.toolbox==0.94