Don't install python-dev on focal

Also, install yamllint in the dib env, as it's a requirement
of dib-lint now but is only in test-requirements for dib.

Change-Id: I083bca901ca51438099d1d3bbbd0076ac3d7da07

nodepool/elements/infra-package-needs/pkg-map

@@ -10,7 +10,8 @@
     "ubuntu": {
       "focal": {
         "ntp": "",
-        "ntpdate": ""
+        "ntpdate": "",
+        "python-dev": ""
       }
    }
   },

tox.ini

@@ -109,6 +109,7 @@ commands = {toxinidir}/specs/generate_specs_site.py {toxinidir}/specs/specs.yaml
 [testenv:dib]
 deps =
    diskimage-builder
+   yamllint
 changedir = {toxinidir}/nodepool
 commands = env ELEMENTS_DIR=elements dib-lint