Drop support for python 3.5

There are now increasing numbers of dependencies like cryptocraphy
that will drop support for Python 3.5 soon. Thus we should consider
dropping the support for Python 3.5 as well.

Change-Id: I830ec3e97cfaac5d336d02755fe788572f36fb07

.zuul.yaml

@@ -232,11 +232,11 @@
         - tox-linters:
             vars:
               tox_install_bindep: false
-        - tox-py35:
+        - tox-py36:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
-            nodeset: ubuntu-xenial
+            nodeset: ubuntu-bionic
             timeout: 4800  # 80 minutes
             vars: &zuul_tox_vars
                 test_setup_environment:
@@ -282,11 +282,11 @@
         - tox-linters:
             vars:
               tox_install_bindep: false
-        - tox-py35:
+        - tox-py36:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
-            nodeset: ubuntu-xenial
+            nodeset: ubuntu-bionic
             timeout: 4800  # 80 minutes
             vars: *zuul_tox_vars
         - tox-py38:

releasenotes/notes/python35-fd8e7f7f577b55e5.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    Support for running Zuul under Python 3.5 has been dropped.

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