only write to stdout buffer when buffer object is available

[thijskramer]

I've stumbled too on the issue mentioned by @Hebarusan in #1382, that Flake8 raises the error: AttributeError: '_io.StringIO' object has no attribute 'buffer' when a Flake8 check fails.

I took a dive in and found that sys.stdout sometimes has the buffer attribute, and sometimes it doesn't.
The Python documentation about stdout states:

When interactive, the stdout stream is line-buffered. Otherwise, it is block-buffered like regular text files.

And also:

[...] However, if you are writing a library (and do not control in which context its code will be executed), be aware that the standard streams may be replaced with file-like objects like io.StringIO which do not support the buffer attribute.

And that seems to be the case!

I've checked this by putting a breakpoint() inside the _write method (the method from which the AttributeError was raised), and printed the result of type(sys.stdout), which was <class '_io.TextIOWrapper'. However, without the breakpoint, it printed <class '_io.StringIO'>.

I therefore concluded that both sys.stdout.buffer.write and sys.stdout.write should work.

[asottile]

dupe #1422