units: 's / m' and 's / m' are not convertible #8260

To paraphrase from PlasmaPy/PlasmaPy#587, exceptions like astropy.units.core.UnitConversionError: 's / m' and 's / m' are not convertible (and a few other cases with first, second and third powers of second over meter, curiously) started popping up in our travis tests as seen at https://travis-ci.org/PlasmaPy/PlasmaPy/jobs/466396211 .

For a brief overview, running python setup.py test -d and getting into plasmapy.physics.distribution.Maxwellian_1D:

(Pdb) distFunc
<Quantity 5.91632969e-07 s / m>
(Pdb) p u.s / u.m
Unit("s / m")
(Pdb) p distFunc
<Quantity 5.91632969e-07 s / m>
(Pdb) p distFunc.unit
Unit("s / m")
(Pdb) p distFunc.to(u.s / u.m)
*** astropy.units.core.UnitConversionError: 's / m' and 's / m' are not convertible

I've managed to figure out that this is localized to astropy 3.1. As suggested by @astrofrog on slack, I ran git bisect with pytest --doctest-modules $PLASMAPY/plasmapy/physics/distribution.py and I was able to identify the first bad commit as

3a478ca29e86144d6c5a0305dde86169a647ff63 is the first bad commit
commit 3a478ca29e86144d6c5a0305dde86169a647ff63
Author: Marten van Kerkwijk <[email protected]>
Date:   Thu Jul 12 21:01:28 2018 -0700

    Further short-cuts for single-unit CompositeUnit initialization.

:040000 040000 1ba7cb9440215ab641197f91ef1623ad9bb39378 5c02db4ede9146ec12f84f57a0382f4900777544 M      astropy

I'm currently trying to find out the exact cause of this bug in that commit, but I figured it's best to submit this issue here before I accidentally close this tab or something.