increase minimum Python to 3.6

Python 3.6 introduces formatted string literals and
flag enums as well as a much faster dict implementation.
These changes make the code so much simpler as to warrant
dropping Python 3.5 support.

Affected distributions are Ubuntu 16.04 and Debian Stretch.

.github/workflows/ci-tests.yml

@@ -126,7 +126,7 @@ jobs:
 
             - uses: actions/setup-python@v2
               with:
-                python-version: 3.5
+                python-version: 3.6
               if: matrix.ubuntu == 18
 
             - uses: ./Nominatim/.github/actions/setup-postgresql

docs/admin/Installation.md

@@ -39,7 +39,7 @@ For running Nominatim:
 
   * [PostgreSQL](https://www.postgresql.org) (9.3+ will work, 11+ strongly recommended)
   * [PostGIS](https://postgis.net) (2.2+)
-  * [Python 3](https://www.python.org/) (3.5+)
+  * [Python 3](https://www.python.org/) (3.6+)
   * [Psycopg2](https://www.psycopg.org) (2.7+)
   * [Python Dotenv](https://github.com/theskumar/python-dotenv)
   * [psutil](https://github.com/giampaolo/psutil)