In numpy/numpy#9883 (part of numpy 1.14) the behavior of the formatting of 0d numpy arrays was changed. This unfortunately broke support for formatting (magnitude + unit) quantities using str.format:

In [2]: "{}".format(3*pq.m)
Out[2]: '3.0'

whereas with an older version of numpy (< 1.14) the unit is included in the formatted string:

In [2]: "{}".format(3*pq.m)
Out[2]: '3.0 m'

My current workaround is to use the !s conversion flag:

In [2]: "{!s}".format(3*pq.m)
Out[2]: '3.0 m'

Apparently unyt doesn't have this problem:
yt-project/unyt#89