ci: rename "trial" tox factor to "twisted"

Ref: https://github.com/pytest-dev/pytest/pull/4848#issuecomment-467909204

tox.ini

@@ -13,7 +13,7 @@ envlist =
     py38
     pypy
     pypy3
-    {py27,py37}-{pexpect,xdist,trial,numpy,pluggymaster}
+    {py27,py37}-{pexpect,xdist,twisted,numpy,pluggymaster}
     py27-nobyte-xdist
     doctesting
     py37-freeze
@@ -67,7 +67,7 @@ basepython = python3
 deps = pre-commit>=1.11.0
 commands = pre-commit run --all-files --show-diff-on-failure
 
-[testenv:py27-trial]
+[testenv:py27-twisted]
 deps =
     {[testenv]deps}
     twisted
@@ -75,9 +75,9 @@ deps =
 commands =
     {env:_PYTEST_TOX_COVERAGE_RUN:} pytest {posargs:testing/test_unittest.py}
 
-[testenv:py37-trial]
-deps = {[testenv:py27-trial]deps}
-commands = {[testenv:py27-trial]commands}
+[testenv:py37-twisted]
+deps = {[testenv:py27-twisted]deps}
+commands = {[testenv:py27-twisted]commands}
 
 [testenv:docs]
 basepython = python3