tox: add generic nobyte and numpy factors

Remove `py27-nobyte` from tox.ini, which was using xdist already.
Therefore this also removes `py27-xdist` from Travis.

"nobyte" was added in 036557ac to test that test_assertrewrite.py works
with a global PYTHONDONTWRITEBYTECODE=1 setting.

"numpy" is only a special dependency, and can be run together with
nobyte/xdist.

.travis.yml

@@ -14,11 +14,10 @@ env:
   matrix:
     - TOXENV=py27 PYTEST_COVERAGE=1
     # Specialized factors for py27.
-    - TOXENV=py27-nobyte PYTEST_COVERAGE=1
-    - TOXENV=py27-xdist PYTEST_COVERAGE=1
+    - TOXENV=py27-nobyte-numpy-xdist PYTEST_COVERAGE=1
     - TOXENV=py27-pluggymaster-xdist
     # Specialized factors for py37.
-    - TOXENV=py37-pexpect,py37-trial,py37-numpy
+    - TOXENV=py37-pexpect,py37-trial
     - TOXENV=py37-pluggymaster-xdist
     - TOXENV=py37-freeze
 
@@ -65,8 +64,8 @@ jobs:
       if: type = cron
 
     - stage: baseline
-      env: TOXENV=py27-pexpect,py27-trial,py27-numpy
-    - env: TOXENV=py37-xdist
+      env: TOXENV=py27-pexpect,py27-trial
+    - env: TOXENV=py37-numpy-xdist
     - env: TOXENV=linting,docs,doctesting
       python: '3.7'