Model.__unicode__() has no effect in Python 3.X

[alx-sdv]

This PR fixes displaying "Schedule object (1)" instead of Schedule.func in django-admin history & messages.
Python 2 support seems to be dropped, so I replaced unicode with str.
Tested on Python 3.8.2, Django 3.1

[codecov-commenter]

Codecov Report

Merging #479 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #479   +/-   ##
=======================================
  Coverage   91.18%   91.18%           
=======================================
  Files          47       47           
  Lines        3051     3051           
=======================================
  Hits         2782     2782           
  Misses        269      269           

Impacted Files	Coverage Δ
django_q/models.py	98.22% <100.00%> (ø)
django_q/tests/test_cluster.py	100.00% <100.00%> (ø)
django_q/tests/test_scheduler.py	100.00% <100.00%> (ø)

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[Koed00]

Thanks!