Dependencies: temporarily set upper limit for psycopg2 (#707)

A recent release of `psycopg2 v2.9` has broken (at least) the
continuous integration tests suite. This was already fixed in
`aiida-core v1.6.x` but not (yet?) backported to the last version
that support Python 3.6 which is still supported by this plugin.

This pin can be removed once a higher version of `aiida-core` is
supported or once `psycopg2` has released a fix for these problems.

setup.json

@@ -93,6 +93,7 @@
         ]
     },
     "install_requires": [
+        "psycopg2-binary<2.9",
         "aiida_core[atomic_tools]~=1.4,>=1.4.4",
         "aiida-pseudo~=0.6.1",
         "jsonschema",