Close the SQLite connection to memory database when the database has been dropped

Instead of relying on the garbage collector to close the connection by deleting
the object, this ensures that the memory database is actually dropped before a
new connection is instantiated and keep the database.

Closes #13796
(grafted from 0a2239fb548b5b20a9ae4d4a8c8c6c34cfed5683)

--HG--
branch : 6.0

trytond/trytond/backend/sqlite/database.py

@@ -441,14 +441,25 @@ class Database(DatabaseInterface):
         return self._conn
 
     def put_connection(self, connection=None, close=False):
-        pass
+        assert connection == self._conn or self._conn is None
+        if self.name == ':memory:':
+            if (self._local.memory_database._conn is None
+                    and connection and close):
+                connection.close()
+                self._conn = None
+        elif close:
+            connection.close()
+            self._conn = None
 
     def close(self):
         if self.name == ':memory:':
-            return
-        if self._conn is None:
-            return
-        self._conn = None
+            if (self._local.memory_database._conn is None
+                    and self._conn):
+                self._conn.close()
+                self._conn = None
+        elif self._conn:
+            self._conn.close()
+            self._conn = None
 
     @classmethod
     def create(cls, connection, database_name):