Add Literal type hinting for if_row_exists arg

[DeflateAwning]

This change improves the type hinting within the library, and also in external uses.

Main change:

• Find: if_row_exists: str
• Replace with: if_row_exists: Literal['ignore', 'update']

Also added the appropriate from typing import ..., Literal

Main benefit: This change allows users to Ctrl+Space in their IDE to see options for that arg.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b91681f) 99.32% compared to head (fe22504) 99.32%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #85   +/-   ##
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  Coverage   99.32%   99.32%           
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  Files          26       26           
  Lines        1923     1923           
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  Hits         1910     1910           
  Misses         13       13           

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[DeflateAwning]

Can you please merge this? It's a very tiny change that would be much-appreciated in modern typed Python.