Ignore error when closing an already closed serial connection

An OSError would be raised when reader and writer threads were
concurrently closing the serial connection. One of them would close
an already closed connection.

Ideally this would be synchronized so we'd be able to detect an issue
closing the connection. In this case a global resource in the system may
be broken rendering the system un-usable.

For now just we just ignore any issues while closing, not taking a
a potential later use of the device into account.

src/numato_gpio/__init__.py

@@ -193,7 +193,11 @@ def cleanup(self):
             self.iodir = self._mask_all_ports
             if self.can_notify:
                 self.notify = False
-            self._ser.close()
+            try:
+                self._ser.close()
+            except OSError:
+                pass
+        self._poll_thread.join()
 
     def write(self, port, value):
         """Write the logic level of a single port.
@@ -406,7 +410,10 @@ def _write(self, query):
             with self._rw_lock:
                 self._ser.write(query)
         except serial.serialutil.SerialException as err:
-            self._ser.close()
+            try:
+                self._ser.close()
+            except OSError:
+                pass
             raise NumatoGpioError("Serial communication failure") from err
 
     def _read_expected_string(self, expected: str) -> None:
@@ -526,7 +533,6 @@ def _poll(self):  # noqa: C901
         """
         try:
             while self._ser and self._ser.is_open:
-
                 if not (b := self._serial_read(1).decode()):
                     continue
 
@@ -540,7 +546,11 @@ def _poll(self):  # noqa: C901
                 self._read_notification()
 
         except (TypeError, serial.serialutil.SerialException):
-            self._ser.close()  # ends the polling loop and its thread
+            # ends the polling loop and its thread
+            try:
+                self._ser.close()
+            except OSError:
+                pass
 
     def _check_port_range(self, port):
         if port not in range(self.ports):