[py] Fix internal error when an unexpected exception is raised while running window_switching_tests (#4654)

This is happening since the switch to use a global driver instance. The
problem is that when a test fails with an exception the
pytest_exception_interact() is called, which quits the driver, and then
the teardown part of the close_windows fixture tries to close the
windows, but window_handles fails because the driver process is no
longer running. We can handle URLError exception when getting the window
handles and simply return without trying to close the windows.

Author: carlosgcampos

py/test/selenium/webdriver/common/window_switching_tests.py

@@ -28,7 +28,19 @@
 def close_windows(driver):
     main_windows_handle = driver.current_window_handle
     yield
-    for handle in driver.window_handles:
+    try:
+        from urllib import request as url_request
+        URLError = url_request.URLError
+    except ImportError:
+        import urllib2 as url_request
+        import urllib2
+        URLError = urllib2.URLError
+
+    try:
+        window_handles = driver.window_handles
+    except URLError:
+        return
+    for handle in window_handles:
         if handle != main_windows_handle:
             driver.switch_to.window(handle)
             driver.close()