Fix password providers not working on Synapse v1.1.0

Fixes a regression introduced during the upgrade to
Synapse v1.1.0 (in 2b3865ceea).

Since Synapse v1.1.0 upgraded to Python 3.7
(https://github.com/matrix-org/synapse/pull/5546),
we need to use a different modules directory when mounting
password provider modules.

roles/matrix-synapse/defaults/main.yml

@@ -49,7 +49,7 @@ matrix_synapse_systemd_required_services_list: ['docker.service']
 # List of systemd services that matrix-synapse.service wants
 matrix_synapse_systemd_wanted_services_list: []
 
-matrix_synapse_in_container_python_packages_path: "/usr/local/lib/python3.6/site-packages"
+matrix_synapse_in_container_python_packages_path: "/usr/local/lib/python3.7/site-packages"
 
 # Specifies which template files to use when configuring Synapse.
 # If you'd like to have your own different configuration, feel free to copy and paste