Fix using file instead of io.IOBase

[martin2250]

As mentioned in #126

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[martin2250]

I signed it!

[googlebot]

CLAs look good, thanks!

[fahhem]

This is not backwards-compatible with Python 2. Please check against file if available. Some options are mentioned here:
https://stackoverflow.com/questions/25738602/how-to-judge-a-file-type-both-work-in-python-2-and-python-3

[martin2250]

Which method do you think is better in this case, using the tuple and try/except or checking hasattr(f, 'fileno') / hasattr(f, 'write')?

[fahhem]

I think fileno() is a good proxy because it supports file-like objects better (like sockets, etc)

[coveralls]

Coverage increased (+0.3%) to 43.893% when pulling 2ead78a on martin2250:master into d9b94b2 on google:master.

[martin2250]

Ok, the PR should be good now