Bump django from 1.8.17 to 2.2.28 in /openstack_requirements/tests/files

Bumps [django](https://github.com/django/django) from 1.8.17 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.8.17...2.2.28)

---
updated-dependencies:
- dependency-name: django
  dependency-type: direct:production
...

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

Author: dependabot[bot]

openstack_requirements/tests/files/gr-base.txt

@@ -8,7 +8,7 @@ cffi
 Cheetah>=2.4.4
 cliff>=1.4
 d2to1>=0.2.10,<0.3
-Django>=1.4,<1.6
+Django>=1.4,<2.3
 django_compressor>=1.4
 dnspython>=1.9.4
 eventlet>=0.12.0

openstack_requirements/tests/files/upper-constraints.txt

@@ -343,7 +343,7 @@ xmltodict===0.10.2
 pyasn1===0.2.3
 python-utils===2.0.1
 oslo.rootwrap===5.5.0
-Django===1.8.17
+Django===2.2.28
 pexpect===4.2.1
 cmd2===0.7.0
 redis===2.10.5