Salt: Add psycopg2 compile dependencies

Starting with psycopg2 version 2.8, the package on pypi no longer
contains a binary version and must be compiled from source. These two
packages are required for this to be possible.

It would have been simpler to just switch to the psycopg2-binary
package, however that isn't a very good solution overall since many
other packages treat "psycopg2" as the dependency that they want
installed, not "psycopg2-binary". Overall, this situation is pretty
messy and I'm not sure what will end up being the final state, but this
should work for now.

More info about the source-only change:
http://initd.org/psycopg/articles/2018/02/08/psycopg-274-released/

salt/salt/python.sls

@@ -22,13 +22,20 @@ venv-setup:
     - require:
       - pkg: python{{ python_version }}-venv
 
+# Packages needed to be able to compile psycopg2 (while installing requirements.txt)
+psycopg2-deps:
+  pkg.installed:
+    - pkgs:
+      - libpq-dev
+      - python{{ python_version }}-dev
+
 pip-installs:
   pip.installed:
     - requirements: {{ app_dir }}/requirements.txt
     - bin_env: {{ venv_dir }}
   require:
     - cmd: venv-setup
-    - pkg: pip-deps
+    - pkg: psycopg2-deps
 
 self-install:
   pip.installed: