Update psycopg2 version in requirements.txt

[lingxiaoyang]

I experienced an issue during Django startup regarding the conflict between psycopg2=2.7.1 (on develop branch) and its dependency glibc 2.26. Need an update of psycopg2 to a version higher than 2.7.3.2.

Or better, use this in requirements.txt (also recommended for other packages):

psycopg2>=2.7,<2.7.99

Refs: psycopg/psycopg2-wheels#2

Building on Ubuntu 18.04, Python 2.7, my error messages were:

1. RuntimeError: populate() isn't reentrant when ./manage.py shell
2. ImportError('/usr/local/lib/python2.7/site-packages/psycopg2/.libs/libresolv-2-c4c53def.5.so: symbol __res_maybe_init version GLIBC_PRIVATE not defined in file libc.so.6 with link time reference',) when looking into it with pdb.

[deepio]

Issue was fixed without closing, current version is psycopg2==2.8.4