Convert two assignments to augmented source code

[elfring]

:eyes: Some source code analysis tools can help to find opportunities for improving software components. :thought_balloon: I propose to increase the usage of augmented assignment statements accordingly.

diff --git a/kartothek/io_components/metapartition.py b/kartothek/io_components/metapartition.py
index 7e4636d..17160f0 100644
--- a/kartothek/io_components/metapartition.py
+++ b/kartothek/io_components/metapartition.py
@@ -1266,7 +1266,7 @@ class MetaPartition(Iterable):
             for df in self.data.values():
 
                 if col in df:
-                    possible_values = possible_values | set(df[col].dropna().unique())
+                    possible_values |= set(df[col].dropna().unique())
                     col_in_partition = True
 
             if (self.label is not None) and (not col_in_partition):
diff --git a/kartothek/utils/predicate_converter.py b/kartothek/utils/predicate_converter.py
index 97b4cfb..27c3af0 100644
--- a/kartothek/utils/predicate_converter.py
+++ b/kartothek/utils/predicate_converter.py
@@ -84,5 +84,5 @@ def convert_predicates_to_cube_conditions(
             "please pass a predicate list with one element."
         )
     for predicate in predicates[0]:
-        condition = condition + (write_predicate_as_cube_condition(predicate),)
+        condition += (write_predicate_as_cube_condition(predicate),)
     return condition