Remove python3.6 testing

This removes python3.6 testing as it shouldn't be needed anymore. The
rest of Zuul is python>=3.8 only at this point. Users on platforms with
older python can run the containers we build to work around that.

We also update the test node choice for python3.8 as Bionic will be EOL
in about 6 months.

Change-Id: I0bbbfe3bea18811599851f5aac025a7484c440b8

.zuul.yaml

@@ -68,11 +68,8 @@
         - tox-linters:
             vars:
               tox_install_bindep: false
-        - tox-py36:
-            nodeset: ubuntu-bionic
-            timeout: 3600
         - tox-py38:
-            nodeset: ubuntu-bionic
+            nodeset: ubuntu-focal
             timeout: 3600
         - tox-py310:
             nodeset: ubuntu-jammy
@@ -85,11 +82,8 @@
         - tox-linters:
             vars:
               tox_install_bindep: false
-        - tox-py36:
-            nodeset: ubuntu-bionic
-            timeout: 3600
         - tox-py38:
-            nodeset: ubuntu-bionic
+            nodeset: ubuntu-focal
             timeout: 3600
         - tox-py310:
             nodeset: ubuntu-jammy

setup.cfg

@@ -26,8 +26,6 @@ classifier =
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
@@ -39,7 +37,7 @@ classifier =
     Topic :: Utilities
 
 [options]
-python-requires = >=3.6
+python-requires = >=3.8
 
 [files]
 packages = zuulclient