Replace deprecated smart_text() with smart_str()

[sp1thas]

The purpose of this PR is to solve #267 by replacing django.utils.encoding.smart_text with django.utils.encoding.smart_str whenever is applicable (Django >= 3.0)

[codecov]

Codecov Report

Merging #295 (938b16e) into master (d77803b) will decrease coverage by 0.29%.
The diff coverage is 66.66%.

❗ Current head 938b16e differs from pull request most recent head 4e7879f. Consider uploading reports for the commit 4e7879f to get more accurate results

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
- Coverage   81.94%   81.65%   -0.30%     
==========================================
  Files          18       18              
  Lines         493      496       +3     
==========================================
+ Hits          404      405       +1     
- Misses         89       91       +2     

Impacted Files	Coverage Δ
auditlog/diff.py	84.61% <66.66%> (-2.49%)	⬇️

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[hramezani]

@sp1thas Thanks for the PR. I include your change into #302.