text_viz.py: Default padding in y depends on whether compact=True/False

This makes text_viz.to_text() work exactly the same as t.to_text().
Which makes more sense to me.

ete4/core/text_viz.py

@@ -13,7 +13,7 @@
 
 
 def to_str(tree, show_internal=True, compact=False, props=None,
-           px=None, py=0, px0=0, cascade=False):
+           px=None, py=None, px0=0, cascade=False):
     """Return a string containing an ascii drawing of the tree.
 
     :param show_internal: If True, show the internal nodes too.