Drop python 3.5 support

We did this in zuul already so drop python 3.5 in nodepool as
well. The py35 job now fails with [1].

[1] ERROR: Package 'keystoneauth1' requires a different Python: 3.5.2 not in '>=3.6'

Change-Id: Ica2a8c5a5c0037190fe23302304de000bd7ad319

.zuul.yaml

@@ -357,8 +357,7 @@
         - nodepool-build-image-siblings
         - zuul-tox-docs
         - tox-pep8
-        - tox-py35:
-            nodeset: ubuntu-xenial
+        - tox-py36
         - tox-py38
         - nodepool-zuul-functional:
             voting: false
@@ -381,8 +380,7 @@
         - nodepool-upload-image
         - zuul-tox-docs
         - tox-pep8
-        - tox-py35:
-            nodeset: ubuntu-xenial
+        - tox-py36
         - tox-py38
         - nodepool-functional-openstack
         - nodepool-functional-openstack-src

doc/source/installation.rst

@@ -37,7 +37,7 @@ Install Nodepool
 
 Install Nodepool prerequisites.
 
-Nodepool requires Python 3.5 or newer.
+Nodepool requires Python 3.6 or newer.
 
 RHEL 7 / CentOS 7::
 

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = Zuul Team
 author-email = zuul-discuss@lists.zuul-ci.org
 home-page = https://zuul-ci.org/
-python-requires = >= 3.5
+python-requires = >= 3.6
 classifier =
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators
@@ -14,7 +14,6 @@ classifier =
     Operating System :: POSIX :: Linux
     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