Add an update_item syntax

[k2bd]

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[codecov-io]

Codecov Report

Attention: Patch coverage is 98.75389% with 4 lines in your changes missing coverage. Please review.

Project coverage is 98.66%. Comparing base (a00c63c) to head (7d570dd).

Files	Patch %	Lines
awstin/dynamodb/orm.py	97.54%	3 Missing ⚠️
awstin/dynamodb/tests/test_update_item.py	99.47%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #61      +/-   ##
===========================================
+ Coverage    98.55%   98.66%   +0.11%     
===========================================
  Files           22       23       +1     
  Lines         1109     1428     +319     
===========================================
+ Hits          1093     1409     +316     
- Misses          16       19       +3     

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