Fix CI setup

We need to assign nodesets that can run the requested python versions
when running tox-pyXY jobs. Also remove the Sphinx version cap as old
sphinx cannot install with modern setuptools. We don't run CI that
includes sphinx so this should be fine.

If this breaks docs builds we can sort that out later.

Change-Id: I577d67912fe6feb68ae50ff34c2622f9461dd122

.zuul.yaml

@@ -13,12 +13,18 @@
     check:
       jobs:
         - gerritlib-jeepyb-integration
-        - tox-py36
-        - tox-py37
-        - tox-py38
+        - tox-py36:
+            nodeset: ubuntu-bionic
+        - tox-py37:
+            nodeset: ubuntu-bionic
+        - tox-py38:
+            nodeset: ubuntu-focal
     gate:
       jobs:
         - gerritlib-jeepyb-integration
-        - tox-py36
-        - tox-py37
-        - tox-py38
+        - tox-py36:
+            nodeset: ubuntu-bionic
+        - tox-py37:
+            nodeset: ubuntu-bionic
+        - tox-py38:
+            nodeset: ubuntu-focal

test-requirements.txt

@@ -1,4 +1,4 @@
-sphinx>=1.1.2,<1.2
+sphinx
 python-subunit
 stestr<3.0.0;python_version<'3.5'
 stestr;python_version>='3.5'