Also simplify the handling of 'panel' in subplot

pygmt/src/subplot.py

@@ -8,7 +8,6 @@
 from pygmt.helpers import (
     build_arg_string,
     fmt_docstring,
-    is_nonstr_iter,
     kwargs_to_strings,
     use_alias,
 )
@@ -172,6 +171,7 @@ def subplot(self, nrows=1, ncols=1, **kwargs):
 @fmt_docstring
 @contextlib.contextmanager
 @use_alias(A="fixedlabel", C="clearance", V="verbose")
+@kwargs_to_strings(panel="sequence_comma")
 def set_panel(self, panel=None, **kwargs):
     r"""
     Set the current subplot panel to plot on.
@@ -221,8 +221,6 @@ def set_panel(self, panel=None, **kwargs):
     {verbose}
     """
     kwargs = self._preprocess(**kwargs)
-    # convert tuple or list to comma-separated str
-    panel = ",".join(map(str, panel)) if is_nonstr_iter(panel) else panel
 
     with Session() as lib:
         arg_str = " ".join(["set", f"{panel}", build_arg_string(kwargs)])