Add trailing comma when a single import doesn't fit on a line. (#504)

Fixes #250.

README.md

@@ -964,6 +964,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
   to be a bad idea (#415)
 
 
+* trailing comma is now added to single imports that don't fit on a line (#250)
+
 
 ### 18.6b4
 

black.py

@@ -2209,7 +2209,7 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
             yield result
 
 
-def right_hand_split(
+def right_hand_split(  # noqa C901
     line: Line, line_length: int, py36: bool = False, omit: Collection[LeafID] = ()
 ) -> Iterator[Line]:
     """Split line into many lines, starting with the last matching bracket pair.
@@ -2250,6 +2250,9 @@ def right_hand_split(
         # the matching `opening_bracket` wasn't available on `line` anymore.
         raise CannotSplit("No brackets found")
 
+    if line.is_import and len(body_leaves) == 1:
+        body_leaves.append(Leaf(token.COMMA, ","))
+
     # Build the new lines.
     for result, leaves in (head, head_leaves), (body, body_leaves), (tail, tail_leaves):
         for leaf in leaves:

tests/data/import_spacing.py

@@ -86,7 +86,7 @@
     Use,
 )
 from name_of_a_company.extremely_long_project_name.component.ttypes import (
-    CuteLittleServiceHandlerFactoryyy
+    CuteLittleServiceHandlerFactoryyy,
 )
 from name_of_a_company.extremely_long_project_name.extremely_long_component_name.ttypes import *