restore compatibility with python 3.7

1 files changed, 4 insertions, 2 deletions

diff --git a/src/hydrilla/builder/build.py b/src/hydrilla/builder/build.py
index 44f3dd9..89c1f5a 100644
--- a/src/hydrilla/builder/build.py
+++ b/src/hydrilla/builder/build.py
@@ -200,11 +200,13 @@ class Build:
         path = piggybacked.resolve_file(desired_path)
         if path is None:
             path = (self.srcdir / desired_path).resolve()
-            if not path.is_relative_to(self.srcdir):
+            try:
+                rel_path = path.relative_to(self.srcdir)
+            except ValueError:
                 raise FileReferenceError(_('loading_{}_outside_package_dir')
                                          .format(filename))
 
-            if str(path.relative_to(self.srcdir)) == 'index.json':
+            if str(rel_path) == 'index.json':
                 raise FileReferenceError(_('loading_reserved_index_json'))
         else:
             include_in_source_archive = False