Fix the doctest for different pandas string dtype representations

Author: seisman

pygmt/datatypes/dataset.py

@@ -239,8 +239,10 @@ def to_pandas(
         1   4.0   5.0   6.0       TEXT4 TEXT567
         2   7.0   8.0   9.0        TEXT8 TEXT90
         3  10.0  11.0  12.0  TEXT123 TEXT456789
-        >>> df.dtypes.to_list()
-        [dtype('float64'), dtype('float64'), dtype('float64'), string[python]]
+        >>> df.dtypes.to_list()[0:3]
+        [dtype('float64'), dtype('float64'), dtype('float64')]
+        >>> df.dtypes["colstr"].name
+        'string'
         """
         vectors = []
         # Deal with numeric columns