Python generally recommends against using `shell=True`
when calling
subprocesses (https://docs.python.org/3/library/subprocess.html#security-considerations).
This also causes issues with shell metacharacters (see
#1469). I'm also
not entirely sure that the shell command *is* fully trustable
- I don't know where {to_filename} and {from_filename} are
from. Rest of nbconvert also prefers using lists instead
of strings to call commands.

svg2pdf also uses a command instead of a string now. We leave
the old string implementation alone for backwards compatibility,
although I'd really prefer to remove it.

We don't need the quotes set up in
#1469,
since using a list automatically deals with that.

…g-list