check_for_fat(): use subprocess.check_output to avoid dependency on Python 3.7

The text=... option for subprocess.Popen was added in Python 3.7 only
("text was added as a more readable alias for universal_newlines"), and
also "encoding" and "errors" were introduced in Python 3.6 only.
We don't want to stick to a specific py3k version, so let's try to
be as backwards compatible as possible.

Since we only need stdout of blkid let's switch to
subprocess.check_output instead.

Thanks: Florian Apolloner for review and feedback