Fix SVG namespace prefixes in HTML emails with attachments

[awdeorio]

Fix issue where inline SVG elements get namespace prefixes (e.g., ns0:svg) when an HTML email template has attachments.

The solution is to replace ElementTree.tostring() with html5lib.serialize() which properly handles HTML5 SVG namespaces. Add regression test for the fix.

Fixes #165

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.74%. Comparing base (f437d83) to head (6c724e5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #170      +/-   ##
===========================================
- Coverage    99.74%   99.74%   -0.01%     
===========================================
  Files            5        5              
  Lines          399      398       -1     
===========================================
- Hits           398      397       -1     
  Misses           1        1              

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[awdeorio]

@jmacdone I think this addresses your issue. If you have a moment to take look, that would be nice.