fix: pip dependency version for psycopg2-binary

Trying to `pip install psycopg2-binary==2.8.3` raised an error for a
long time. Since version `2.8.4` is recent and also seem to work I
suggest updating the requirements so other users dont run into the
following error, where pip tries to use `pg_config` for the BINARY
package:

Collecting psycopg2-binary==2.8.3 (from -r contrib/pyln-testing/requirements.txt (line 6))
  Using cached 91911be018dd876c1e0f12e1b5fb90/psycopg2-binary-2.8.3.tar.gz
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-zapr0fhs/psycopg2-binary/setup.py'"'"'; __file__='"'"'/tmp/pip-install-zapr0fhs/psycopg2-binary/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base pip-egg-info
         cwd: /tmp/pip-install-zapr0fhs/psycopg2-binary/
    Complete output (23 lines):
    running egg_info
    creating pip-egg-info/psycopg2_binary.egg-info
    writing pip-egg-info/psycopg2_binary.egg-info/PKG-INFO
    writing dependency_links to pip-egg-info/psycopg2_binary.egg-info/dependency_links.txt
    writing top-level names to pip-egg-info/psycopg2_binary.egg-info/top_level.txt
    writing manifest file 'pip-egg-info/psycopg2_binary.egg-info/SOURCES.txt'

    Error: pg_config executable not found.

    pg_config is required to build psycopg2 from source.  Please add the directory
    containing pg_config to the $PATH or specify the full executable path with the
    option:

        python setup.py build_ext --pg-config /path/to/pg_config build ...

    or with the pg_config option in 'setup.cfg'.

    If you prefer to avoid building psycopg2 from source, please install the PyPI
    'psycopg2-binary' package instead.

    For further information please check the 'doc/src/install.rst' file (also at
    <http://initd.org/psycopg/docs/install.html>).

    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Changelog-None

contrib/pyln-testing/requirements.txt

@@ -3,4 +3,4 @@ Flask==1.1.1
 cheroot==6.5.5
 ephemeral-port-reserve==1.1.1
 python-bitcoinlib==0.10.1
-psycopg2-binary==2.8.3
+psycopg2-binary==2.8.4

tests/requirements.txt

@@ -10,4 +10,4 @@ pytest-xdist==1.29.0
 python-bitcoinlib==0.10.2
 tqdm==4.32.2
 pytest-timeout==1.3.3
-psycopg2-binary==2.8.3
+psycopg2-binary==2.8.4