Switch ugettext to ugettext_lazy in models.py

[haikuginger]

Fixes #288.

We have a use case where we need to import this package before the app registry is ready. This pull request changes ugettext calls in models.py to ugettext_lazy.

If we can get this in a 1.9.1 release soon, that'd be great, but if not, then we can just point at the master hash once it's merged.

@itsjeyd, can you check this code for me?

[codecov-io]

Codecov Report

Merging #301 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #301   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files          15       15           
  Lines         590      590           
  Branches       72       72           
=======================================
  Hits          576      576           
  Misses          7        7           
  Partials        7        7

Impacted Files	Coverage Δ
simple_history/models.py	96.85% <100%> (ø)	⬆️

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[itsjeyd]

@haikuginger The code LGTM. I'm currently building a new devstack for testing the changes; will formally approve once I've verified the changes according to the instructions you left on the internal ticket.

[itsjeyd]

@haikuginger 👍

• I tested this: Ran paver update_db with version 1.9.0 of django-simple-history installed; it failed with

  django.core.exceptions.AppRegistryNotReady: The translation infrastructure cannot be initialized before the apps registry is ready. Check that you don't make non-lazy gettext calls at import time.

  (as expected). Then I installed the version from this PR and ran paver update_db again. This time it did not produce an AppRegistryNotReady error.

• I read through the code

• I checked for accessibility issues N/A

• Includes documentation: In the form of a changelog entry.