test_replaygain: fix complicated and incorrect exception handling

This is an incorrect translation of a python 2 reraise to python 3.
With python 3, however, we can just rely on exception chaining to get
the traceback, so get rid of the complicated re-raising entirely, with
the additional benefit that the exception from the tear-down is also
shown.

test/test_replaygain.py

@@ -55,18 +55,8 @@ class ReplayGainCliTestBase(TestHelper):
         try:
             self.load_plugins('replaygain')
         except Exception:
-            import sys
-            # store exception info so an error in teardown does not swallow it
-            exc_info = sys.exc_info()
-            try:
-                self.teardown_beets()
-                self.unload_plugins()
-            except Exception:
-                # if load_plugins() failed then setup is incomplete and
-                # teardown operations may fail. In particular # {Item,Album}
-                # may not have the _original_types attribute in unload_plugins
-                pass
-            raise None.with_traceback(exc_info[2])
+            self.teardown_beets()
+            self.unload_plugins()
 
         album = self.add_album_fixture(2)
         for item in album.items():