Run with list args

[gvalkov]

Hello,

This PR adds the following API change to the run() function:

# Works with strings.
run('my command')

# Also works with lists.
run(['my', 'command'])

# All arguments are safely quoted.
run(['my', '$(ls /etc)', 'command with spaces'])  
# runs: "my '$(ls /etc)' 'command with spaces'"

This is of course, quite similar to the way subprocess.Popen and os.execv handle arguments by default. I think using a list is more convenient when running more complex commands, as you then don't have to worry about construction a safe string to pass to run.

Another possibility (that might be taking this a bit too far) is to make command a vararg. For example:

# These are equivalent.
run('my', 'command')  # This quotes arguments for us.
run('my command')

The six version bump was necessary for the six.moves.shlex_quote function, which is in different places in Py2 and Py3.

Kind regards,
Georgi

[gvalkov]

str|list is what seems to fail CI - I have to figure out what syntax sphinx expects here.

[gvalkov]

Unfortunately, sphinx cannot resolve more than one type in a param or type directive. Since inv sites make sphinx-build treat warnings as errors, we end up with a broken build.

If you consider this patch for merging, I'll look into how this issue can be worked around (or fixed).