python 3.6 SQLite compat vacuum in transaction

fixes sqlite3.OperationalError: cannot VACUUM from within a transaction
solution from https://bugs.python.org/issue28518

src/gpodder/dbsqlite.py

@@ -55,9 +55,9 @@ class Database(object):
         self.commit()
 
         with self.lock:
-            cur = self.cursor()
-            cur.execute("VACUUM")
-            cur.close()
+            self.db.isolation_level = None
+            self.db.execute('VACUUM')
+            self.db.isolation_level = ''
 
         self._db.close()
         self._db = None

src/gpodder/minidb.py

@@ -66,7 +66,9 @@ class Store(object):
 
     def close(self):
         with self.lock:
+            self.db.isolation_level = None
             self.db.execute('VACUUM')
+            self.db.isolation_level = ''
             self.db.close()
 
     def _register(self, class_):