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

This reverts commit b3394953.

stack/slapos.cfg

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