pretty: fix xobj for even height

Previously pprint({x**2: 1}) was failing with cryptic exception

    ValueError: xobj: expect length = 2*k+1

And this is a wrong approach.

Things need to just work, so I've modified xobj to tweak evenly heighted
objects somewhat. After all it looks good, so why not?

In [1]: [x**2]
Out[1]:
⎡ 2⎤
⎣x ⎦

In [2]: (x**2,)
Out[2]:
⎛ 2⎞
⎝x ⎠

In [3]: {x**2: 1}
Out[3]:
⎧ 2   ⎫
⎨x : 1⎬
⎩     ⎭

diff --git a/sympy/printing/pretty/pretty_symbology.py b/sympy/printing/pretty/pretty_symbology.py
--- a/sympy/printing/pretty/pretty_symbology.py
+++ b/sympy/printing/pretty/pretty_symbology.py
@@ -311,7 +311,10 @@ def xobj(symb, length):
     if bot is None:  bot = ext
     if mid is not None:
         if (length % 2) == 0:
-            raise ValueError('xobj: expect length = 2*k+1')
+            # even height, but we have to print it somehow anyway...
+            # XXX is it ok?
+            length += 1
+
     else:
         mid = ext
 
diff --git a/sympy/printing/tests/test_pretty_unicode.py b/sympy/printing/tests/test_pretty_unicode.py
--- a/sympy/printing/tests/test_pretty_unicode.py
+++ b/sympy/printing/tests/test_pretty_unicode.py
@@ -198,3 +198,30 @@ u"""\
     assert u == s
 
 
+def test_upretty_seq_even():
+    """there used to be a bug when pprinting sequences with even height"""
+    u = upretty([x**2])
+    s = \
+u"""\
+⎡ 2⎤
+⎣x ⎦\
+"""
+    assert u == s
+
+    u = upretty((x**2,))
+    s = \
+u"""\
+⎛ 2⎞
+⎝x ⎠\
+"""
+    assert u == s
+
+    u = upretty({x**2: 1})
+    s = \
+u"""\
+⎧ 2   ⎫
+⎨x : 1⎬
+⎩     ⎭\
+"""
+    assert u == s
+