Test remaining bits

[jodal]

This PR takes the project's line test coverage to 100%. The test suite is
configured to fails if this is not maintained with future changes.

• Test Cursor.get_page() with a non-existant page
• Test various ways of opening and closing the client
• Test 404 and 410 responses
• Test handling of non-HTTP errors
• Ensure all type annotations works on Python 3.8
• Require the current full test coverage to be maintained

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (94d681d) 95.82% compared to head (b6306f8) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #18      +/-   ##
===========================================
+ Coverage   95.82%   100.00%   +4.17%     
===========================================
  Files           8         8              
  Lines         359       356       -3     
  Branches       22        21       -1     
===========================================
+ Hits          344       356      +12     
+ Misses         10         0      -10     
+ Partials        5         0       -5     

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