Bump psycopg2 version (#9204)

As we use `execute_values` with the `fetch` parameter.

changelog.d/9204.misc

@@ -0,0 +1 @@
+Bump minimum `psycopg2` version.

synapse/python_dependencies.py

@@ -86,8 +86,8 @@ REQUIREMENTS = [
 
 CONDITIONAL_REQUIREMENTS = {
     "matrix-synapse-ldap3": ["matrix-synapse-ldap3>=0.1"],
-    # we use execute_batch, which arrived in psycopg 2.7.
-    "postgres": ["psycopg2>=2.7"],
+    # we use execute_values with the fetch param, which arrived in psycopg 2.8.
+    "postgres": ["psycopg2>=2.8"],
     # ACME support is required to provision TLS certificates from authorities
     # that use the protocol, such as Let's Encrypt.
     "acme": [

tox.ini

@@ -117,7 +117,7 @@ commands =
     # Make all greater-thans equals so we test the oldest version of our direct
     # dependencies, but make the pyopenssl 17.0, which can work against an
     # OpenSSL 1.1 compiled cryptography (as older ones don't compile on Travis).
-    /bin/sh -c 'python -m synapse.python_dependencies | sed -e "s/>=/==/g" -e "s/psycopg2==2.6//" -e "s/pyopenssl==16.0.0/pyopenssl==17.0.0/" | xargs -d"\n" pip install'
+    /bin/sh -c 'python -m synapse.python_dependencies | sed -e "s/>=/==/g" -e "/psycopg2/d" -e "s/pyopenssl==16.0.0/pyopenssl==17.0.0/" | xargs -d"\n" pip install'
 
     # Install Synapse itself. This won't update any libraries.
     pip install -e ".[test]"