Newer version of mypy fixes buggy error when trying to update only boto3 stubs.
However it brings new checks and starts to yell when we index into
cusror.fetchone without checking for None first. So this introduces a wrapper
to simplify quering for scalar values. I tried to use cursor_factory connection
argument but without success. There can be a better way to do that,
but this looks the simplest
* Add a test case for reading historic page versions
Test read_page_at_lsn returns correct results when compared to page inspect.
Validate possiblity of reading pages from dropped relation.
Ensure funcitons read latest version when null lsn supplied.
Check that functions do not poison buffer cache with stale page versions.
