Introduce importlib.util.ModuleManager which is a context manager to
handle providing (and cleaning up if needed) the module to be loaded.

A future commit will use the context manager in
Lib/importlib/_bootstrap.py and thus why the code is placed there
instead of in Lib/importlib/util.py.
diff --git a/Lib/importlib/util.py b/Lib/importlib/util.py
index 1316437..f817a40 100644
--- a/Lib/importlib/util.py
+++ b/Lib/importlib/util.py
@@ -1,5 +1,6 @@
 """Utility code for constructing importers, etc."""
 
+from ._bootstrap import ModuleManager
 from ._bootstrap import module_for_loader
 from ._bootstrap import set_loader
 from ._bootstrap import set_package