Add py310 job in gate

As 3.10 is next python version we will be testing in
next cycle (after Zed), let's add it in our testing in advance.

Change-Id: I1d60c21473c7b657a3f4ad57525c4561f2cfcfd5

bindep.txt

@@ -5,7 +5,6 @@ libffi-dev [platform:dpkg]
 libffi-devel [platform:rpm]
 gcc [platform:rpm]
 gcc [platform:dpkg]
-python-dev [platform:dpkg]
 python-devel [platform:rpm]
 python3-dev [platform:dpkg]
 python3-devel [platform:rpm]

zuul.d/project.yaml

@@ -10,6 +10,7 @@
         - openstack-tox-pep8
         - openstack-tox-py38
         - openstack-tox-py39
+        - openstack-tox-py310
         - tempest-full-parallel:
             # Define list of irrelevant files to use everywhere else
             irrelevant-files: &tempest-irrelevant-files
@@ -131,6 +132,7 @@
         - openstack-tox-pep8
         - openstack-tox-py38
         - openstack-tox-py39
+        - openstack-tox-py310
         - tempest-slow-py3:
             irrelevant-files: *tempest-irrelevant-files
         - neutron-ovs-grenade-multinode: