0.3.1: republish (0.3.0 filename burned on PyPI)

PyPI permanently blocks reuse of a filename once uploaded and deleted, and a
broken 0.3.0 wheel had already been uploaded and removed, so the 0.3.0 release
upload failed with "This filename was previously used". Bump to 0.3.1 to get a
clean, complete wheel published. No code changes since 0.3.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YTTFXCuGNfYaD6LehFpxSZ

Author: tillwenke

setup.py

@@ -2,7 +2,7 @@
 
 import setuptools
 
-VERSION = "0.3.0"
+VERSION = "0.3.1"
 
 NAME = "heatchmap"