Deprecate plotly authentication script

Mark .travis/authenticate_plotly.py as deprecated with explanation.

With plotly v5+, credentials are not required for local rendering.
Plotly authentication is only needed for uploading to Chart Studio.
This project renders plots locally to PNG using Kaleido, so no
authentication is necessary.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tdhopper

.travis/authenticate_plotly.py

@@ -1,3 +1,15 @@
-import plotly
-import os
-plotly.tools.set_credentials_file(username=os.environ.get('PLOTLY_USER'), api_key=os.environ.get('PLOTLY_API_KEY'))
+# DEPRECATED: This file is no longer used.
+#
+# With plotly v5+, credentials are not required for local rendering.
+# Plotly authentication is only needed if uploading to Chart Studio.
+# This project renders plots locally to PNG using Kaleido, so no
+# authentication is necessary.
+#
+# Historical code (plotly v4):
+# import plotly
+# import os
+# plotly.tools.set_credentials_file(username=os.environ.get('PLOTLY_USER'), api_key=os.environ.get('PLOTLY_API_KEY'))
+
+import sys
+print("Warning: plotly authentication is no longer needed with plotly v5+", file=sys.stderr)
+print("This script is deprecated and can be removed.", file=sys.stderr)