Add mktmpdir method to PicardTestCase, and use it more widely

test/test_bytes2human.py

@@ -1,6 +1,4 @@
 import os.path
-import shutil
-import tempfile
 
 from test.picardtestcase import PicardTestCase
 
@@ -12,8 +10,7 @@ class Testbytes2human(PicardTestCase):
     def setUp(self):
         super().setUp()
         # we are using temporary locales for tests
-        self.tmp_path = tempfile.mkdtemp(suffix=self.__class__.__name__)
-        self.addCleanup(shutil.rmtree, self.tmp_path)
+        self.tmp_path = self.mktmpdir()
         self.localedir = os.path.join(self.tmp_path, 'locale')
 
     def test_00(self):