Removed -xdist on travis envs py3{4,6,8} and pypy{3} since they do not exist in tox.ini and are failing with InterpreterNotFound errors

Author: fermezz

.travis.yml

@@ -49,12 +49,12 @@ jobs:
       python: '3.7'
 
     # Coverage tracking is slow with pypy, skip it.
-    - env: TOXENV=pypy-xdist
+    - env: TOXENV=pypy
       python: 'pypy'
-    - env: TOXENV=pypy3-xdist
+    - env: TOXENV=pypy3
       python: 'pypy3'
 
-    - env: TOXENV=py34-xdist
+    - env: TOXENV=py34
       python: '3.4'
     - env: TOXENV=py35-xdist
       python: '3.5'
@@ -80,7 +80,7 @@ jobs:
     - env: TOXENV=py37-freeze
 
     # Jobs only run via Travis cron jobs (currently daily).
-    - env: TOXENV=py38-xdist
+    - env: TOXENV=py38
       python: '3.8-dev'
       if: type = cron
 
@@ -90,7 +90,7 @@ jobs:
       env: TOXENV=py27-pexpect,py27-twisted PYTEST_COVERAGE=1
       python: '2.7'
     # Use py36 here for faster baseline.
-    - env: TOXENV=py36-xdist
+    - env: TOXENV=py36
       python: '3.6'
     - env: TOXENV=linting,docs,doctesting PYTEST_COVERAGE=1
       cache:
@@ -123,8 +123,8 @@ jobs:
 
 matrix:
   allow_failures:
-    - python: '3.8-dev'
-      env: TOXENV=py38-xdist
+    - env: TOXENV=py38
+      python: '3.8-dev'
     # Temporary (https://github.com/pytest-dev/pytest/pull/5334).
     - env: TOXENV=pypy3-xdist
       python: 'pypy3'