When looping over a joined str, if a node is ast.Str and the value is
just a newline "\n", the write() function adds an additional indentation
after it, which fails to represent the original string. By calling
self.result.append() here directly the issue is resolved.

The added test could show the issue. With code_gen unmodifed, it fails
with the following error:

```
AssertionError: "if 1:\n    x = f'{host}\\n\\t{port}\\n    '" != "if
1:\n    x = f'{host}\\n\\t{port}\\n'"
  if 1:
      -     x = f'{host}\n\t{port}\n    '?
      ----
      +     x = f'{host}\n\t{port}\n'
```

Which is exactly the problem.

This fixes parsing issues with many of Python 3.7's stdlib. Namely the
following ones:

/usr/lib/python3.7/warnings.py
/usr/lib/python3.7/netrc.py
/usr/lib/python3.7/test/test_embed.py
/usr/lib/python3.7/test/support/testresult.py
/usr/lib/python3.7/idlelib/grep.py

Fixes #119