Pyflakes generating error on interpolated string

Pyflakes reports errors on interpolated string syntax.

```
print("")
print(f"") # Error reported here
```

I believe the problem is related to whether Pyflakes run with Python 3 vs Python 2. See log below where it behaves differently when run with Python3 even when same version is reported.

Should ide-python be running Pyflakes with Python3?

My system, atom, and ide-python are all up to date. And I have reinstalled pyflakes to ensure latest version in python3.

```
4pi:flask juan$ python -m pyflakes --version
2.1.1 Python 2.7.15 on Darwin
4pi:flask juan$ python -m pyflakes foo.py
foo.py:2:9: invalid syntax
print(f"")
^
4pi:flask juan$ python3 -m pyflakes --version
2.1.1 Python 3.7.3 on Darwin
4pi:flask juan$ python3 -m pyflakes foo.py
4pi:flask juan$
```

首先使用所示的 foo.py 示例重现报告，并检查 ide-python 如何调用 Pyflakes。比较 issue 中的 Python 2 和 Python 3 命令；当插值字符串能够使用 Python 3 进行检查且不会产生错误的语法错误时，即表示完成。

由索引模型根据 Issue 内容生成。