Bump psycopg2 from 2.7.3 to 2.8.3

Bumps [psycopg2](https://github.com/psycopg/psycopg2) from 2.7.3 to 2.8.3.
- [Release notes](https://github.com/psycopg/psycopg2/releases)
- [Changelog](https://github.com/psycopg/psycopg2/blob/master/NEWS)
- [Commits](https://github.com/psycopg/psycopg2/commits)

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

requirements.txt

@@ -13,7 +13,7 @@ pytest==3.2.0
 pytest-cov==2.5.1
 pylint==2.3.0
 flake8==3.7.7
-psycopg2==2.7.3
+psycopg2==2.8.3
 pycountry==1.20
 python-dateutil==2.6.1
 rauth==0.7.3