style Seguir las directrices de estilo de código de Django

* Use single quotes for strings, or a double quote if the string contains a single quote.
* An exception to PEP 8 is our rules on line lengths. Don’t limit lines of code to
  79 characters if it means the code looks significantly uglier or is harder to read.
  We allow up to 119 characters as this is the width of GitHub code review;
  anything longer requires horizontal scrolling which makes review more difficult.

Ver <https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/coding-style/>
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Name Stage Failure
failed
test Test
Running with gitlab-runner 12.8.0 (1b659122)
on estudiosformacion fa353a65
Using Shell executor...
Running on estudiosformacion...
ERROR: Job failed: exit status 1