Update stylable_admin_list.py to fix models.FieldDoesNotExist exception

[vesnikos]

a quick PL to fix models.FieldDoesNotExist exception that is happening with django 3.1:

FieldDoesNotExist should be imported from exceptions now, as it's no longer referenced at django.db.model anymore? I think it should retain compatibility with django2

[codecov]

Codecov Report

Merging #80 into master will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master      #80   +/-   ##
=======================================
  Coverage   64.96%   64.96%           
=======================================
  Files          12       12           
  Lines         762      762           
=======================================
  Hits          495      495           
  Misses        267      267           

Impacted Files	Coverage Δ
...lymorphic_tree/templatetags/stylable_admin_list.py	18.58% <33.33%> (ø)

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[vdboor]

thanks, this was merged some time ago!