requirements: Switch to openapi-core 0.13.4

In commit b7f3c3d ("tests: Switch to openapi-core 0.13.x") we added
support for 'openapi-core' 0.13.x. However, we needed to use a commit
from master since an important fix [1] was not included in the latest
release at the time, 0.13.3. 0.13.4 has since been released so let's
move on and use that.

[1] python-openapi/openapi-core#226

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

requirements-test.txt

@@ -3,4 +3,4 @@ psycopg2-binary~=2.8.0
 sqlparse~=0.4.0
 python-dateutil~=2.8.0
 tblib~=1.7.0
-https://github.com/p1c2u/openapi-core/archive/97ec8c796746f72ef3298fe92078b5f80e1f66f7.tar.gz
+openapi-core~=0.13.4