refactor: make Reason enum values instances of str

[federicobond]

This PR

Is a refinement on #126. In particular, it makes Reason values also instances of str, and thus directly comparable without having to access the .value property, and without having to define custom getters and setters in dataclasses.

The motivation for this was to fix a no-redef issue reported by mypy in the FlagEvaluationDetails definition:

open_feature/flag_evaluation/flag_evaluation_details.py:19: error: Name "reason" already defined on line 15  [no-redef]

StrEnum is available since Python 3.11 so it requires a minimal backport for earlier Python versions.

[codecov]

Codecov Report

Merging #142 (1e9e37f) into main (695da28) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #142   +/-   ##
=======================================
  Coverage   93.26%   93.26%           
=======================================
  Files          21       22    +1     
  Lines         386      386           
=======================================
  Hits          360      360           
  Misses         26       26           

Flag	Coverage Δ
unittests	93.26% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...feature/flag_evaluation/flag_evaluation_details.py	100.00% <ø> (ø)
open_feature/_backports/strenum.py	100.00% <100.00%> (ø)
open_feature/flag_evaluation/reason.py	100.00% <100.00%> (ø)

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[hlipsig]

I don't have any concerns here. This is a good change.