Add py3 tests to tox with folders that work

It's just a few tests, but it will at least prevent a few files from
regressing. Also, it makes it easiert to check your code against py36
while writing it.

Signed-off-by: Adrian Tschira <nota@notafile.com>

tox.ini

 [tox]
-envlist = packaging, py27, pep8
+envlist = packaging, py27, py36 pep8
 
 [testenv]
 deps =
@@ -46,6 +46,14 @@ commands =
 # )
 usedevelop=true
 
+[testenv:py36]
+usedevelop=true
+commands =
+    /usr/bin/find "{toxinidir}" -name '*.pyc' -delete
+    coverage run {env:COVERAGE_OPTS:} --source="{toxinidir}/synapse" \
+        "{envbindir}/trial" {env:TRIAL_FLAGS:} {posargs:tests/metrics} {env:TOXSUFFIX:}
+    {env:DUMP_COVERAGE_COMMAND:coverage report -m}
+
 [testenv:packaging]
 deps =
     check-manifest