LITE HMRC v1.4.0

What's Changed

• ICMSLST-1537 Use gunicorn for serving in production by @davidwtbuxton in #126
• Fix CI pipeline failures by @davidwtbuxton in #133
• ICMSLST-1530 Use DRF serializers for validating API requests by @davidwtbuxton in #130
• ICMSLST-1547 Backport Django's Choices/enum type for use with enums by @davidwtbuxton in #131
• ICMSLST-1570 Use positional arguments to logging calls by @davidwtbuxton in #132
• ICMSLST-1595 More defaults for environment variables by @davidwtbuxton in #135
• ICMSLST-1597 Add gunicorn to Python dependencies by @davidwtbuxton in #136
• ICMSLST-1578 Build CHIEF messages for other systems (e.g. ICMS) by @davidwtbuxton in #134
• ICMSLST-1634 Remove hard-coded path from the LITE_API_URL setting (ag… by @davidwtbuxton in #138
• ICMSLST-1627 Catch invalid licence data by @davidwtbuxton in #137

Full Changelog: v1.3.0...1.4.0