Add python-requires entry indicating 3.5 is required

We require 3.5 now but we didn't add the metadata. Add it.

While OpenStack broadly only supports 3.6, openstacksdk
has a wider audience, including nodepool which still uses
3.5. So add the 3.5 unit tests back.

Use lower-constraints for py35 unit tests because the
upper-constraints have versions of things that have
droppped py35 support already.

Depends-On: https://review.opendev.org/#/c/715467/
Change-Id: I680e0bab5ba3af7be5e914ece64c6bd25d1f2191

.zuul.yaml

@@ -393,6 +393,7 @@
     templates:
       - check-requirements
       - openstack-lower-constraints-jobs
+      - openstack-python35-jobs
       - openstack-python3-ussuri-jobs
       - openstacksdk-functional-tips
       - openstacksdk-tox-tips

releasenotes/notes/python-3.5-629817cec092d528.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    openstacksdk does not test or support python2 as of 0.40,
+    but the releases have still accidentally worked (except for
+    0.44 which was broken for python2). We're now explicitly
+    marking releases as requiring >= 3.5 so that things don't
+    attempt to install something that's bound to be broken.

setup.cfg

@@ -14,8 +14,10 @@ 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
+python-requires = >=3.5
 
 [files]
 packages =

tox.ini

@@ -127,3 +127,7 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[testenv:py35]
+basepython = python3.5
+deps = {[testenv:lower-constraints]deps}