Manager.imports(): Restore Python import behavior even if an exception is raised

The context manager used to only restore sys.metapath and sys.modules if no exception was raised inside the imports() context.

I've also tried to add a few tests for the behavior. Unfortunately, they seem to be failing on JPype1==0.6 and I don't know enough about JPype to understand why. Some help on this front would be appreciated.

cmmnbuild_dep_manager/tests/test_cmmnbuild_dep_manager.py

@@ -184,3 +184,36 @@ def test_class_path_join(mocked_mgr):
 
     with unittest.mock.patch('os.name', 'something-else'):
         assert mgr.class_path(extras) == "module1.jar:path/to/mod2.jar"
+
+
+def test_imports_removed_on_success(mocked_mgr):
+    pytest.importorskip('jpype', minversion='1.0')
+    mgr = mocked_mgr[0]
+    with mgr.imports():
+        import java.util
+        assert java.util in sys.modules.values()
+    assert java.util not in sys.modules.values()
+
+def test_imports_removed_on_failure(mocked_mgr):
+    pytest.importorskip('jpype', minversion='1.0')
+    mgr = mocked_mgr[0]
+    with pytest.raises(ValueError):
+        with mgr.imports():
+            import java.util
+            assert java.util in sys.modules.values()
+            raise ValueError()
+    assert java.util not in sys.modules.values()
+
+def test_imports_are_reentrant(mocked_mgr):
+    pytest.importorskip('jpype', minversion='1.0')
+    mgr = mocked_mgr[0]
+    with mgr.imports():
+        import java.util
+        with mgr.imports():
+            import java.lang
+            assert java.util in sys.modules.values()
+            assert java.lang in sys.modules.values()
+        assert java.util in sys.modules.values()
+        assert java.lang not in sys.modules.values()
+    assert java.util not in sys.modules.values()
+    assert java.lang not in sys.modules.values()