Fix DeprecationWarning with use of datetime.utcnow() on Python 3.12

Replace deprecated datetime.datetime.utcnow() with datetime.datetime.now(tz=datetime.UTC).

Author: weiji14

pygmt/tests/test_clib_put_vector.py

@@ -1,8 +1,8 @@
 """
 Test the functions that put vector data into GMT.
 """
+import datetime
 import itertools
-from datetime import datetime
 
 import numpy as np
 import numpy.testing as npt
@@ -122,7 +122,7 @@ def test_put_vector_string_dtype():
             "2021-02-03T00:00:00",
             "2021-02-03T04:00:00",
             "2021-02-03T04:05:06",
-            f"{datetime.utcnow().strftime('%Y-%m-%d')}T04:50:06",
+            f"{datetime.datetime.now(tz=datetime.UTC).strftime('%Y-%m-%d')}T04:50:06",
         ],
     ]