Use tempfile.TemporaryDirectory in conform/glibcconform.py.

Now that we require Python 3.4 or later, Python code creating
temporary directories can use tempfile.TemporaryDirectory in "with" to
have the directory deleted automatically instead of needing to use
try/finally to handle removing a directory created with
tempfile.mkdtemp.  This patch does so in conform/glibcconform.py.

Tested for x86_64.

	* conform/glibcconform.py: Do not import shutil.
	(list_exported_functions): Use tempfile.TemporaryDirectory instead
	of mkdtemp.