Fix SyntaxError from non-raw regular expression

[effigies]

It seems that invalid escape sequences will at some undetermined point in the future stop emitting SyntaxWarnings and start emitting SyntaxErrors (see the second bullet item in https://docs.python.org/3/whatsnew/3.12.html#other-language-changes).

On the whole, svgutils seems not to need changes to be usable on new versions of Python, but this warning is no longer safe to ignore.

Only the first change is really necessary, but I used pyupgrade --py36-plus to just make safe upgrades. I only included conversions away from %-formatted strings if they actually made the line shorter.