iotests: Replace deprecated ConfigParser.readfp()

iotest 277 fails on Fedora 33 (Python 3.9) because a deprecation warning
changes the output:

    nbd-fault-injector.py:230: DeprecationWarning: This method will be
    removed in future versions.  Use 'parser.read_file()' instead.

In fact, readfp() has already been deprecated in Python 3.2 and the
replacement has existed since the same version, so we can now
unconditionally switch to read_file().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201113100602.15936-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>