fix: actually call fromisoformat()

[jennifer-richards]

Fixes tastypie API breakage introduced in #9330

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.73%. Comparing base (86f2cfa) to head (c2cc4dc).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/api/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9399      +/-   ##
==========================================
- Coverage   88.74%   88.73%   -0.01%     
==========================================
  Files         320      320              
  Lines       41810    41810              
==========================================
- Hits        37103    37100       -3     
- Misses       4707     4710       +3     

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