Move over to using assertRaises as a context manager for importlib tests.

Obviously one shouldn't do whole sale conversions like this, but I was already
going through the test code and I was bored at the airport.
diff --git a/Lib/importlib/test/source/test_file_loader.py b/Lib/importlib/test/source/test_file_loader.py
index 0384e7d..b18750c 100644
--- a/Lib/importlib/test/source/test_file_loader.py
+++ b/Lib/importlib/test/source/test_file_loader.py
@@ -93,7 +93,8 @@
                 file.write('+++ bad syntax +++')
             loader = _bootstrap._PyPycFileLoader('_temp', mapping['_temp'],
                                                     False)
-            self.assertRaises(SyntaxError, loader.load_module, name)
+            with self.assertRaises(SyntaxError):
+                loader.load_module(name)
             for attr in attributes:
                 self.assertEqual(getattr(orig_module, attr), value)
 
@@ -104,7 +105,8 @@
                 file.write('=')
             loader = _bootstrap._PyPycFileLoader('_temp', mapping['_temp'],
                                                     False)
-            self.assertRaises(SyntaxError, loader.load_module, '_temp')
+            with self.assertRaises(SyntaxError):
+                loader.load_module('_temp')
             self.assertTrue('_temp' not in sys.modules)
 
 
@@ -166,8 +168,8 @@
                 bytecode_file.write(imp.get_magic())
                 bytecode_file.write(source_timestamp)
                 bytecode_file.write(b'AAAA')
-            self.assertRaises(ValueError, self.import_, mapping['_temp'],
-                                '_temp')
+            with self.assertRaises(ValueError):
+                self.import_(mapping['_temp'], '_temp')
             self.assertTrue('_temp' not in sys.modules)