use f-string instead of .format syntax

tests/test_dataset.py

@@ -225,19 +225,19 @@ class Create1001TestCase(unittest.TestCase):
 class BulkIOTestCase(unittest.TestCase):
     def testOpen(self):
         for fn in fns:
-            with self.subTest(msg="Opening test file {:s}".format(fn)):
+            with self.subTest(msg=f"Opening test file {str(fn)}"):
                 ict      = icartt.Dataset(fn, loadData=False)
                 self.assertEqual( type(ict), icartt.Dataset )
 
     def testReadData(self):
         for fn in fns:
-            with self.subTest(msg="Reading data from test file {:s}".format(fn)):
+            with self.subTest(msg=f"Reading data from test file {str(fn)}"):
                 ict      = icartt.Dataset(fn, loadData=True)
                 self.assertEqual( type(ict), icartt.Dataset )
 
     def testWriteHeader(self):
         for fn in fns:
-            with self.subTest(msg="Writing header for test file {:s}".format(fn)):
+            with self.subTest(msg=f"Writing header for test file {str(fn)}"):
                 ict      = icartt.Dataset(fn, loadData=False)
 
                 strIn  = open(fn)
@@ -249,7 +249,7 @@ class BulkIOTestCase(unittest.TestCase):
 
     def testWrite(self):
         for fn in fns:
-            with self.subTest(msg="Writing data for test file {:s}".format(fn)):
+            with self.subTest(msg=f"Writing data for test file {str(fn)}"):
                 ict      = icartt.Dataset(fn, loadData=True)
 
                 strIn  = open(fn)