bpo-30980: Fix double close in asyncore.file_wrapper (#2789) (#2898)

* bpo-30980: Fix close test to fail

test_close_twice was not considering the fact that file_wrapper is
duping the file descriptor. Closing the original descriptor left the
duped one open, hiding the fact that close protection is not effective.

* bpo-30980: Fix double close protection

Invalidated self.fd before closing, handling correctly the case when
os.close raises.

* bpo-30980: Fix fd leak introduced in the fixed test

Lib/asyncore.py

@@ -619,8 +619,9 @@ if os.name == 'posix':
         def close(self):
             if self.fd < 0:
                 return
-            os.close(self.fd)
+            fd = self.fd
             self.fd = -1
+            os.close(fd)
 
         def fileno(self):
             return self.fd