Replace django.conf.urls.url with django.urls.re_path

[samueljsb]

This avoid a deprecation warning: django.conf.urls.url is an alias for django.urls.re_path but will be removed in Django 4.0.

This method was only added in Django 2.0, so we have to check whether we are using a version of Django greater than 2.0 before we import it. Otherwise, we use the old url method.

Fixes #68

[codecov-commenter]

Codecov Report

Merging #69 (b4144e9) into master (0ff60e7) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #69   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           51        54    +3     
=========================================
+ Hits            51        54    +3     

Impacted Files	Coverage Δ
pwa/urls.py	100.00% <100.00%> (ø)

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[samueljsb]

No longer needed -- this was done by #88.