Test tox.requires doesn't break sibling installs

Update the alternate tox.ini files in test-playbooks/python to
trigger configuration we saw expose a bug in the siblings lib's
showconfig parser when a tox.requires value resulted in pip
activity. Update its minversion to the earliest tox which provides
the tox.requires configuration option, for clarity.

While we're there, switch to using tox_config_file instead of
overriding zuul_work_dir since that's now an option for this role
and was previously not exercised in the test job.

Change-Id: I521580da978903cd116c872447b1c028c470e63d

test-playbooks/python/tox.ini

@@ -1,6 +1,9 @@
 [tox]
+minversion = 3.2
 envlist = linters
 skipsdist = true
+# Test that we don't break when the requires feature is used
+requires = tox-bindep
 
 [testenv]
 whitelist_externals = sh

test-playbooks/python/tox.yaml

@@ -80,7 +80,8 @@
       include_role:
         name: tox
       vars:
-        zuul_work_dir: "{{ zuul.project.src_dir }}/test-playbooks/python/"
+        # Make sure the tox_config_file option works
+        tox_config_file: test-playbooks/python
         tox_envlist: '%s'
         # Make sure we test with verbose output even if the default changes
         tox_extra_args: '-vv'