Resolve deprecation of django.utils.decorators.available_attrs in Django 2.0

[browniebroke]

One of the Python 2 compatibility API, from Django 3.0 release notes:

django.utils.decorators.available_attrs() - This function returns functools.WRAPPER_ASSIGNMENTS.

[codecov]

Codecov Report

Merging #85 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #85   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines          368       379   +11     
=========================================
+ Hits           368       379   +11     

Impacted Files	Coverage Δ
django_codemod/visitors/__init__.py	100.00% <100.00%> (ø)
django_codemod/visitors/decorators.py	100.00% <100.00%> (ø)

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