Use gettext_lazy for Python 3 code

[noisecapella]

I upgraded a project of ours to Python 3.8 and started seeing this deprecation warning: django.utils.deprecation.RemovedInDjango40Warning: django.utils.translation.ugettext_lazy() is deprecated in favor of django.utils.translation.gettext_lazy().. According to the Django docs, ugettext_lazy and gettext_lazy are interchangable for Python 3. This PR should address that deprecation warning by keeping ugettext_lazy for Python 2.x and using gettext_lazy in Python 3.

[codecov]

Codecov Report

Merging #99 into master will increase coverage by 1.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   80.71%   81.87%   +1.16%     
==========================================
  Files           7        7              
  Lines         140      149       +9     
  Branches       16       19       +3     
==========================================
+ Hits          113      122       +9     
  Misses         20       20              
  Partials        7        7

Impacted Files	Coverage Δ
src/robots/forms.py	78.57% <100%> (+5.84%)	⬆️
src/robots/models.py	84.21% <100%> (+1.35%)	⬆️
src/robots/admin.py	100% <100%> (ø)	⬆️

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[jezdez]

Thanks!