Fix or suppress Psalm 6 issues

Author: trowski

psalm.xml

@@ -14,6 +14,12 @@
     </projectFiles>
 
     <issueHandlers>
+        <ClassMustBeFinal>
+            <errorLevel type="suppress">
+                <file name="src/PostgresQueryError.php"/>
+            </errorLevel>
+        </ClassMustBeFinal>
+
         <MethodSignatureMismatch>
             <errorLevel type="suppress">
                 <file name="src/Internal/AbstractHandle.php"/>
@@ -33,6 +39,12 @@
             </errorLevel>
         </PossiblyUnusedMethod>
 
+        <PossiblyUnusedProperty>
+            <errorLevel type="suppress">
+                <file name="src/PostgresNotification.php"/>
+            </errorLevel>
+        </PossiblyUnusedProperty>
+
         <RiskyTruthyFalsyComparison>
             <errorLevel type="suppress">
                 <directory name="src"/>
@@ -44,5 +56,11 @@
                 <directory name="src"/>
             </errorLevel>
         </UnsupportedPropertyReferenceUsage>
+
+        <UnusedClass>
+            <errorLevel type="suppress">
+                <directory name="src"/>
+            </errorLevel>
+        </UnusedClass>
     </issueHandlers>
 </psalm>