Remove blacklisted py3 xen tests

Xen UT failures seem to have cleared up, which may or may not have
anything to do with [1]. Remove the blacklist file and get these back
in the py3 jobs.

[1] https://review.openstack.org/#/c/591061/

Change-Id: Iacc1ae01f535caed64793d57b757ea07a8c46620

Author: Eric Fried

tests-py3.txt

@@ -43,7 +43,7 @@ commands =
 basepython = python3.5
 commands =
   {[testenv]commands}
-  stestr run --blacklist-file=tests-py3.txt {posargs}
+  stestr run {posargs}
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
 
 [testenv:py36]
@@ -268,4 +268,4 @@ deps =
   -r{toxinidir}/requirements.txt
 commands =
   {[testenv]commands}
-  stestr run --blacklist-file=tests-py3.txt {posargs}
+  stestr run {posargs}