Fixes #38

Fixes #131

Fixes #136

Fixes #138

Closes #126

Fixes #127

* Fix code generation with attributes of integer literals
* Handle Constant.kind since it was only added in Python 3
* Remove allow_failures for Python 3.8 from .travis.yml

Fixes #120

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

Fixes #124

Fixes #116

Closes #108

Travis CI has outdated version of Python 3.7.0, so '3.7-dev'
was removed for now.

This was tested against CPython commit 1d419faccdaae27d968cef1e1ece6b9374694573.

Closes #106

Fixes #85
Fixes #100

This fixes the change in python API introduced by
https://bugs.python.org/issue29463

Fixes #101.

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.

This commit also replaces pypy3.3-5.2-alpha1 tox/travis with pypy3.5.

The former PyPy version has a bug causing the tests to fail:

https://bitbucket.org/pypy/pypy/issues/2316/osget_terminal_size-is-missing-on-pypy3-hg

Fixes #89.

The domain astor.rtfd.org doesn't matched the certificate that is being served. so the documentation link resulted in a browser warning, rather than the docs.

Fixes #86

Fixes #87