tools not loaded for Gromacs 2016/2018 in Python 3 on macOS

[Oliver Beckstein]

When using Gromacs 2016/2018 (the gmx command) and Python 3 (and apparently only on macOS) then the tools are not correctly loaded; only tools.Gmx appears.

Reason: byte string snafu

            out = subprocess.check_output([driver, '-quiet', 'help',
                                           'commands'])
            for line in str(out).encode('ascii').splitlines()[5:-1]:

The subprocess.check_output() returns a b string,

>>> type(out)
bytes

which, when converted with str(), does not maintain the newlines but rather includes them quoted
and then splitlines() does not work anymore.

[Oliver Beckstein]

This was introduced in PR #127 but not caught because we are only testing on Linux.