Declare support for Python3.8

Replace our tox-py37 job with tox-py38, extend the list of trove
classifiers for Python versions in package metadata, and replace the
"py35" in the tox.ini envlist with just "py3" so that folks running
`tox` unqualified on their systems will use whatever python3
interpreter they have on hand (odds are it's in our supported range
these days). Also uncap python-daemon so we use a version compatible
with Python >=3.8.

Change-Id: I4b382bb89847d3ae2b20292d923c3c2211dfb254

.zuul.yaml

@@ -216,7 +216,7 @@
                     ZUUL_TEST_ROOT: /tmp/zuul-test
                 tox_environment:
                     ZUUL_TEST_ROOT: /tmp/zuul-test
-        - tox-py37:
+        - tox-py38:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
@@ -273,7 +273,7 @@
                     ZUUL_TEST_ROOT: /tmp/zuul-test
                 tox_environment:
                     ZUUL_TEST_ROOT: /tmp/zuul-test
-        - tox-py37:
+        - tox-py38:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*

requirements.txt

@@ -8,7 +8,7 @@ github3.py>=1.1.0
 PyYAML>=3.1.0
 paramiko>=2.0.1
 GitPython>=2.1.8
-python-daemon>=2.0.4,<2.1.0
+python-daemon>=2.0.4
 extras
 statsd>=3.0
 voluptuous>=0.10.2

setup.cfg

@@ -15,6 +15,9 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
 
 [pbr]
 warnerrors = True

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.2
 skipsdist = True
-envlist = pep8,py35{-docker}
+envlist = pep8,py3{-docker}
 ignore_basepython_conflict = True
 # TODO(stephenfin): It would be good to set 'requires = tox-docker', but doing
 # so borks the tools/pip.sh wrapper we're using here and probably isn't what