test(python): Update exception imports in test suite

[stinodego]

In a lot of cases, exceptions were already imported from the exceptions module directly. Now we do this consistently across the test suite, instead of using the top-level re-exports (I want to get rid of those - more info in later PR).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.01%. Comparing base (a62ab75) to head (cd20366).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17035      +/-   ##
==========================================
+ Coverage   80.98%   81.01%   +0.02%     
==========================================
  Files        1448     1448              
  Lines      190551   190551              
  Branches     2723     2723              
==========================================
+ Hits       154322   154366      +44     
+ Misses      35726    35682      -44     
  Partials      503      503              

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