Bump freezegun from 0.3.12 to 0.3.13

Bumps [freezegun](https://github.com/spulec/freezegun) from 0.3.12 to 0.3.13.
- [Release notes](https://github.com/spulec/freezegun/releases)
- [Changelog](https://github.com/spulec/freezegun/blob/master/CHANGELOG)
- [Commits](spulec/freezegun@0.3.12...0.3.13)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -2,5 +2,5 @@ coverage==5.0.3
 Django==1.11.26 # pyup: <2.0
 django-debug-toolbar==2.1
 docutils==0.16
-freezegun==0.3.12
+freezegun==0.3.13
 django-formtools==2.2