TODOs before beta release

[liangyuanruo]

TODOs before beta release

• Shore up readme documentation
• Align exports (remove top-level functions)
• Align type signatures with SDK (e.g. exceptions vs None)
• Look into removing typing extensions
• File github issues for TODOs (e.g. test coverage)
• Publish on PyPI under OGP creds (pypi-publish@open.gov.sg?)