Bump version to v3.1.8

This version adds compatibility with resvg/usvg v0.42.0

gerbolyze/__init__.py

@@ -21,7 +21,7 @@ import click
 
 import gerbonara as gn
 
-__version__ = '3.1.7'
+__version__ = '3.1.8'
 
 @click.group()
 def cli():

svg-flatten/include/gerbolyze.hpp

@@ -31,7 +31,7 @@
 
 namespace gerbolyze {
 
-    constexpr char lib_version[] = "3.1.7";
+    constexpr char lib_version[] = "3.1.8";
 
     typedef std::function<std::vector<d2p> *(double, double, double)> sampling_fun;