Drop support for Python 3.5

An upcoming change needs a feature from Git 2.10, which Ubuntu 16.04
LTS lacks. Since Python 3.5 reached end of life with the 3.5.10
release on 2020-09-05, it's been unsupported by its maintainers for
nearly 2 years anyway.

Change-Id: I0369c29baa344c4dc76eaa47b3467b6117eb933c

.zuul.yaml

@@ -7,8 +7,6 @@
       jobs: &jobs
         - build-python-release
         - tox-linters
-        - tox-py35:
-            nodeset: ubuntu-xenial
         - tox-py36:
             nodeset: ubuntu-bionic
         - tox-py37:

releasenotes/notes/drop-py35-a078f1c6f3c4fff4.yaml

@@ -0,0 +1,3 @@
+---
+upgrade:
+  - Python 3.5 is no longer supported.

setup.cfg

@@ -23,14 +23,13 @@ classifiers =
     License :: OSI Approved :: Apache Software License
     Operating System :: OS Independent
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python
     Topic :: Utilities
-python_requires = >= 3.5
+python_requires = >= 3.6
 
 [files]
 packages =