Fix mixed content by updating Google Fonts link to HTTPS

[hwahyeon]

The browser displayed the following warning:

Mixed Content: The page at 'https://biopython.org/' was loaded over HTTPS, but requested an insecure stylesheet 'http://fonts.googleapis.com/css?family=PT+Sans%3A400,400italic,700%7CAbril+Fatface'. This request has been blocked; the content must be served over HTTPS.

The Google Fonts link has been updated to use HTTPS to resolve this issue.

Verified locally:

• Font request to https://fonts.googleapis.com/css?family=PT+Sans:400,400italic,700|Abril+Fatface returns 200 OK
• Styles load as expected (confirmed in Styles panel)