Trac #27126: py3: Fix plot module for python3

The root cause for matplotlib's {{{AttributeError: Unknown property
which}}} was testing {{{hasattr(x, '__iter__')}}} to filter {{{str}}}.
{{{str}}} hasn't an {{{__iter__}}} method in py2 but has one in py3.

URL: https://trac.sagemath.org/27126
Reported by: vklein
Ticket author(s): Vincent Klein
Reviewer(s): Travis Scrimshaw

src/sage/plot/graphics.py

@@ -2383,10 +2383,10 @@ def _get_vmin_vmax(self, vmin, vmax, basev, axes_pad):
         at least 10 units apart::
 
             sage: p = Graphics()
-            sage: p._get_vmin_vmax(1, 2, 10, None)
-            (9/10, 10.0)
-            sage: p._get_vmin_vmax(1, 5, 10, None)
-            (9/10, 10.0)
+            sage: p._get_vmin_vmax(1, 2, 10, None) == (9/10, 10)
+            True
+            sage: p._get_vmin_vmax(1, 5, 10, None) == (9/10, 10)
+            True
             sage: p._get_vmin_vmax(1, 10, 10, None)
             (9/10, 11)
             sage: p._get_vmin_vmax(1, 11, 10, None)
@@ -2903,7 +2903,7 @@ def matplotlib(self, filename=None,
             if vgridlines=='minor':
                 vgridstyle['which']='both'
 
-            if hasattr(hgridlines, '__iter__'):
+            if not isinstance(hgridlines, str) and hasattr(hgridlines, '__iter__'):
                 hlines=iter(hgridlines)
                 hgridstyle.pop("minor",None)
                 for hline in hlines:
@@ -2919,7 +2919,7 @@ def matplotlib(self, filename=None,
                 if hgridlines not in (None, False):
                     subplot.yaxis.grid(True, **hgridstyle)
 
-            if hasattr(vgridlines, '__iter__'):
+            if not isinstance(vgridlines, str) and hasattr(vgridlines, '__iter__'):
                 vlines=iter(vgridlines)
                 vgridstyle.pop("minor",None)
                 for vline in vlines:

src/sage/plot/histogram.py

@@ -92,8 +92,8 @@ def get_minmax_data(self):
             DeprecationWarning: the 'normed' option is deprecated. Use 'density' instead.
             See https://trac.sagemath.org/25260 for details.
             sage: h.get_minmax_data()
-            doctest:warning ...:
-            VisibleDeprecationWarning: Passing `normed=True` on non-uniform bins has always been broken, and computes neither the probability density function nor the probability mass function. The result is only correct if the bins are uniform, when density=True will produce the same result anyway. The argument will be removed in a future version of numpy.
+            doctest:warning ...
+            ...VisibleDeprecationWarning: Passing `normed=True` on non-uniform bins has always been broken, and computes neither the probability density function nor the probability mass function. The result is only correct if the bins are uniform, when density=True will produce the same result anyway. The argument will be removed in a future version of numpy.
             {'xmax': 10.0, 'xmin': 3.0, 'ymax': 0.476190476190..., 'ymin': 0}
         """
         import numpy