Add default line length for jupyter (Qiskit#1400)

When running the text circuit drawer in jupyter the default line length
was being setting with the get_terminal_size() call. However this was
returning the size of the terminal used to launch jupyter, not the cell
size. So this could cause issues if your terminal is much smaller or
much larger than the cell size. To fix this issue this commit adds a
default of 80 characters for the default line length when running in
jupyter so it'll always fit in the cell.

qiskit/tools/visualization/_circuit_visualization.py

@@ -82,8 +82,9 @@ def circuit_drawer(circuit,
         line_length (int): Sets the length of the lines generated by `text`
             output type. This useful when the drawing does not fit in the
             console. If None (default), it will try to guess the console width
-            using shutil.get_terminal_size(). If you don't want pagination at
-            all, set `line_length=-1`.
+            using shutil.get_terminal_size(). However, if you're running in
+            jupyter the default line length is set to 80 characters. If you
+            don't want pagination at all, set `line_length=-1`.
         reverse_bits (bool): When set to True reverse the bit order inside
             registers for the output visualization.
         plot_barriers (bool): Enable/disable drawing barriers in the output

qiskit/tools/visualization/_text.py

@@ -10,6 +10,7 @@
 """
 
 from shutil import get_terminal_size
+import sys
 
 from ._error import VisualizationError
 
@@ -460,7 +461,10 @@ def lines(self, line_length=None):
         if line_length is None:
             line_length = self.line_length
         if line_length is None:
-            line_length, _ = get_terminal_size()
+            if ('ipykernel' in sys.modules) and ('spyder' not in sys.modules):
+                line_length = 80
+            else:
+                line_length, _ = get_terminal_size()
 
         noqubits = len(self.qregs)
         layers = self.build_layers()