Update mockito version from 2.24.0 to 3.8.0 (#436)

pom.xml

@@ -51,7 +51,7 @@
 
         <junit.version>5.4.0</junit.version>
         <hamcrest.version>2.1</hamcrest.version>
-        <mockito.version>2.24.0</mockito.version>
+        <mockito.version>3.8.0</mockito.version>
         <cobertura.version>2.7</cobertura.version>
         <coveralls.version>3.1.0</coveralls.version>