Move systemd-python to an optional logging subpackage

MANIFEST.in

 include Makefile
 include requirements.txt
-include requirements-swh.txt
-include requirements-http.txt
 include requirements-db.txt
+include requirements-http.txt
+include requirements-logging.txt
+include requirements-swh.txt
 include version.txt
 recursive-include swh/core/sql *.sql
 recursive-include swh py.typed

requirements-logging.txt

+systemd-python

requirements.txt

 Deprecated
 PyYAML
-systemd-python

setup.py

@@ -51,7 +51,8 @@ setup(
     install_requires=parse_requirements(None, 'swh'),
     setup_requires=['vcversioner'],
     extras_require={
-        'testing': parse_requirements('test', 'db', 'http'),
+        'testing': parse_requirements('test', 'db', 'http', 'logging'),
+        'logging': parse_requirements('logging'),
         'db': parse_requirements('db'),
         'http': parse_requirements('http'),
     },

tox.ini

@@ -4,6 +4,7 @@ envlist=flake8,mypy,py3-{core,db,server}
 [testenv:py3-core]
 deps =
   -rrequirements-test.txt
+  -rrequirements-logging.txt
   .
 commands =
   pytest --doctest-modules {envsitepackagesdir}/swh/core/tests {posargs}