Bump django from 1.11.18 to 1.11.21 in /backend

[dependabot[bot]]

Bumps django from 1.11.18 to 1.11.21.

Commits

- [`bc1f79d`](https://github.com/django/django/commit/bc1f79d0a01d085500aa82ff29800403291a91a4) [1.11.x] Bumped version for 1.11.21 release. - [`c238701`](https://github.com/django/django/commit/c238701859a52d584f349cce15d56c8e8137c52b) [1.11.x] Fixed CVE-2019-12308 -- Made AdminURLFieldWidget validate URL before... - [`4b3716e`](https://github.com/django/django/commit/4b3716e6540e06874b9053b59e39cd76fb48b677) [1.11.x] Added stub release notes for security releases. - [`331d765`](https://github.com/django/django/commit/331d76528154407927f88759c9e520494e29b914) [1.11.x] Refs [#27807](https://github-redirect.dependabot.com/django/django/issues/27807) -- Removed docs for User.username_validator. - [`d13490c`](https://github.com/django/django/commit/d13490c18a075799aec1951ee7e260876b5d1ba6) [1.11.x] Refs [#30331](https://github-redirect.dependabot.com/django/django/issues/30331) -- Doc'd that psycopg2 < 2.8 is required. - [`22c0564`](https://github.com/django/django/commit/22c0564193dd8dbd2e6cbfba1cd2b11d334bf172) [1.11.x] Fixed [#30277](https://github-redirect.dependabot.com/django/django/issues/30277) -- Fixed broken links to packaging.python.org. - [`9530fac`](https://github.com/django/django/commit/9530fac978073af8b3c6b374b588873d5c18d2a4) [1.11.x] Fixed serializers test crash if PyYAML isn't installed. - [`f8ce3cd`](https://github.com/django/django/commit/f8ce3cd1622e974aeb88691cd5761db096513d7d) [1.11.x] Fixed serializers tests for PyYAML 5.1+. - [`f13bfde`](https://github.com/django/django/commit/f13bfdeb559c533ce5ace19a250821e6f8abb13d) [1.11.x] Reverted "Fixed relative paths imports per isort 4.3.5." - [`b9beb6a`](https://github.com/django/django/commit/b9beb6a52e84e565533f029555eea731d92fe4f3) [1.11.x] Fixed relative paths imports per isort 4.3.5. - Additional commits viewable in [compare view](https://github.com/django/django/compare/1.11.18...1.11.21)

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[daniviga]

@dependabot use these assignees

[dependabot[bot]]

OK, from now on I won't assign anyone for Python pull requests on this repo.

[daniviga]

@dependabot use these reviewers

[dependabot[bot]]

OK, from now on I'll ask for review from @nastasi-oq for Python pull requests on this repo.