Added support for Django 3.1

[smithdc1]

• FieldDoesNotExist has moved to django.core.exceptions
• ugettest_lazy() deprected if favor our gettext_lazy
• url() is deprecated in favor of path() and re_path()
• Added django3.1 to test matrix

[codecov[bot]]

Codecov Report

Merging #124 (4063c9b) into master (9437c16) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #124   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          395       395           
  Branches        59        59           
=========================================
  Hits           395       395           

Impacted Files	Coverage Δ
django_hosts/apps.py	100.00% <100.00%> (ø)
django_hosts/managers.py	100.00% <100.00%> (ø)

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