[yoga] Ensure get_requests_for_local_unit doesn't fail on incomplete relation

This is a rebuild/make sync for charms to pickup the fix in charmhelpers to fix
any inadvertant accesses of ['ca'] in the relation data before it is available
from vault in the certificates relation.  Fix in charmhelpers is in [1].

[1] https://github.com/juju/charm-helpers/pull/827
Closes-Bug: #2028683

Change-Id: I9c5c3776f00775a87217c817aa134dfc1d1e34c8

rebuild

@@ -2,4 +2,4 @@
 # when dependencies of the charm change,
 # but nothing in the charm needs to.
 # simply change the uuid to something new
-53bf780c-1178-11ec-bc16-bb24856e98b0
+acca317c-3b6e-11ee-98c6-7bc9c4342aa8

tox.ini

@@ -23,6 +23,7 @@ requires =
   pip < 20.3
   virtualenv < 20.0
   setuptools<50.0.0
+  tox<4.0
 
 # NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
 minversion = 3.18.0
@@ -91,7 +92,8 @@ commands = stestr run --slowest {posargs}
 [testenv:pep8]
 basepython = python3
 deps = flake8==3.9.2
-       charm-tools==2.8.3
+       PyYAML==6.0.1
+       charm-tools==2.8.6
 commands = flake8 {posargs} src unit_tests
 
 [testenv:func-target]