fix: avoid mutables as default arguments. Compute date default arguments at runtime rather than loadtime.

[rjsparks]

Fixes #4141

[codecov]

Codecov Report

Merging #4144 (b559986) into main (7de5d93) will increase coverage by 0.00%.
The diff coverage is 94.44%.

@@           Coverage Diff           @@
##             main    #4144   +/-   ##
=======================================
  Coverage   88.21%   88.21%           
=======================================
  Files         295      295           
  Lines       39164    39201   +37     
=======================================
+ Hits        34548    34581   +33     
- Misses       4616     4620    +4     

Impacted Files	Coverage Δ
ietf/api/serializer.py	76.02% <60.00%> (-0.63%)	⬇️
ietf/nomcom/utils.py	91.17% <66.66%> (-0.23%)	⬇️
ietf/doc/utils.py	87.58% <100.00%> (+0.08%)	⬆️
ietf/meeting/views.py	91.15% <100.00%> (+0.01%)	⬆️
ietf/person/templatetags/person_filters.py	79.48% <100.00%> (+1.10%)	⬆️
ietf/review/utils.py	89.55% <100.00%> (+0.04%)	⬆️
ietf/stats/views.py	88.79% <100.00%> (+0.09%)	⬆️
ietf/utils/mail.py	79.57% <100.00%> (+0.49%)	⬆️
ietf/utils/test_smtpserver.py	81.25% <100.00%> (ø)
ietf/group/utils.py	93.69% <0.00%> (-0.91%)	⬇️
... and 3 more

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