cbook.is_sequence_of_strings knows string objects

**Issue**
`cbook.is_sequence_of_strings` cannot tell of a pandas series
is contains strings. Pandas series are numpy arrays of type
`object`.

**Problem**
`cbook.is_sequence_of_strings` uses `cbook.is_string_like` to dismiss
strings as not sequences of strings. However, numpy string arrays
of `dtype=object` are string-like since they support string
concatenation.

**Solution**
Do not dismiss an `ndarray` if it appears string-like, go ahead and
peak inside.

doc/users/whats_new/cbook.rst

@@ -0,0 +1,16 @@
+cbook.is_sequence_of_strings recognizes string objects
+``````````````````````````````````````````````````````
+
+This is primarily how pandas stores a sequence of strings.
+
+    >>> import pandas as pd
+    >>> import matplotilb.cbook as cbook
+    >>> a = np.array(['a', 'b', 'c'])
+    >>> cbook.sequence_of_strings(a)
+    False
+    >>> a = np.array(['a', 'b', 'c'], dtype=object)
+    >>> cbook.sequence_of_strings(a)
+    True
+    >>> s = pd.Series(['a', 'b', 'c])]
+    >>> cbook.sequence_of_strings(s)
+    True

lib/matplotlib/cbook.py

@@ -783,7 +783,7 @@ def is_sequence_of_strings(obj):
     """
     if not iterable(obj):
         return False
-    if is_string_like(obj):
+    if is_string_like(obj) and not isinstance(obj, np.ndarray):
         return False
     for o in obj:
         if not is_string_like(o):

lib/matplotlib/tests/test_cbook.py

@@ -26,6 +26,23 @@ def test_is_string_like():
     assert cbook.is_string_like("hello world")
     assert_equal(cbook.is_string_like(10), False)
 
+    y = ['a', 'b', 'c']
+    assert_equal(cbook.is_string_like(y), False)
+
+    y = np.array(y)
+    assert_equal(cbook.is_string_like(y), False)
+
+    y = np.array(y, dtype=object)
+    assert cbook.is_string_like(y)
+
+
+def test_is_sequence_of_strings():
+    y = ['a', 'b', 'c']
+    assert cbook.is_sequence_of_strings(y)
+
+    y = np.array(y, dtype=object)
+    assert cbook.is_sequence_of_strings(y)
+
 
 def test_restrict_dict():
     d = {'foo': 'bar', 1: 2}