fix(deps): update dependency pygit2 to v1.18.1

Author: renovate[bot]

pyproject.toml

@@ -45,8 +45,8 @@ dependencies = [
   'opentype-sanitizer',
   'vttlib',
   # pygit2 1.16.0 only supports 3.10+
-  'pygit2==1.15.0; python_version < "3.13"',
-  'pygit2==1.16.0; python_version >= "3.13"',
+  'pygit2==1.18.1; python_version < "3.13"',
+  'pygit2==1.18.1; python_version >= "3.13"',
   'strictyaml',
   'fontmake[json]>=3.3.0',
   'skia-pathops',