Fix trailing newlines in f-strings

If an f-string was ended with a newline character (more precisely, if
the last element of a JoinedStr node was just a newline character), its
round-trip source code representation would have an extra newline on a
wrong place, resulting in invalid syntax.

The bug affects only python>=3.6 (f-strings weren't supported before)

This commit fixes the aforementioned bug.

Fixes berkerpeksag#89.

astor/code_gen.py

@@ -580,6 +580,9 @@ def recurse(node):
 
             index = len(result)
             recurse(node)
+
+            # Flush trailing newlines (so that they are part of mystr)
+            self.write('')
             mystr = ''.join(result[index:])
             del result[index:]
             self.colinfo = res_index, str_index  # Put it back like we found it

docs/changelog.rst

@@ -22,6 +22,12 @@ New features
 
 .. _`Issue 86`: https://github.com/berkerpeksag/astor/issues/86
 
+Bug fixes
+~~~~~~~~~
+
+* Fix function, class and module docstrings for Python 3.7
+
+  Starting Python 3.7.0 the docstrings `are stored <https://bugs.python.org/issue29463>`_ in a new ``docstring`` attribute of the AST node instead of the first expression in code body. This change wasn't taken into account in ``astor.code_gen``, which resulted in ``astor.code_gen.to_source`` omitting the docstring in Python 3.7+.
 
 0.6.2 - 2017-11-11
 ------------------

tests/test_code_gen.py

@@ -476,6 +476,15 @@ def test_non_string_leakage(self):
         '''
         self.assertAstRoundtrips(source)
 
+    def test_fstring_trailing_newline(self):
+        """
+        Check that trailing newlines in f-strings are printed correctly
+        """
+        source = '''
+        x = f"""{host}\n\t{port}\n"""
+        '''
+        self.assertSrcRoundtripsGtVer(source, (3, 6))
+
 
 if __name__ == '__main__':
     unittest.main()