refactor: adjust amazon forwarding validation function

[firstof9]

Proposed change

Adjust amazon forwarding email validation function.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Documentation update
• Adds a new shipper
• Update existing shipper

Additional information

• This PR is related to issue: ISSUE: amazon_fwd returns "value should be a string" on existing valid entry #937

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.49%. Comparing base (8608d22) to head (8ef2f61).
Report is 29 commits behind head on dev.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #957      +/-   ##
==========================================
- Coverage   95.07%   93.49%   -1.59%     
==========================================
  Files           8        8              
  Lines        1482     1567      +85     
==========================================
+ Hits         1409     1465      +56     
- Misses         73      102      +29     

Files	Coverage Δ
custom_components/mail_and_packages/config_flow.py	93.13% <100.00%> (-6.87%)	⬇️

... and 3 files with indirect coverage changes