CI: use py37 instead of py36 by default

Closes https://github.com/pytest-dev/pytest/issues/4370.

.travis.yml

@@ -8,7 +8,7 @@ stages:
 - name: deploy
   if: repo = pytest-dev/pytest AND tag IS present
 python:
-  - '3.6'
+  - '3.7'
 install:
   - pip install --upgrade --pre tox
 env:
@@ -18,10 +18,11 @@ env:
     - TOXENV=py27-nobyte
     - TOXENV=py27-xdist
     - TOXENV=py27-pluggymaster PYTEST_NO_COVERAGE=1
-    # Specialized factors for py36.
-    - TOXENV=py36-pexpect,py36-trial,py36-numpy
-    - TOXENV=py36-xdist
-    - TOXENV=py36-pluggymaster PYTEST_NO_COVERAGE=1
+    # Specialized factors for py37.
+    - TOXENV=py37-pexpect,py37-trial,py37-numpy
+    - TOXENV=py37-xdist
+    - TOXENV=py37-pluggymaster PYTEST_NO_COVERAGE=1
+    - TOXENV=py37-freeze PYTEST_NO_COVERAGE=1
 
 jobs:
   include:
@@ -31,10 +32,7 @@ jobs:
       dist: trusty
     - env: TOXENV=py35
       python: '3.5'
-    - env: TOXENV=py36-freeze PYTEST_NO_COVERAGE=1
-      python: '3.6'
     - env: TOXENV=py37
-      python: '3.7'
     - &test-macos
       language: generic
       os: osx
@@ -56,7 +54,9 @@ jobs:
     - env: TOXENV=py34
       python: '3.4'
     - env: TOXENV=py36
+      python: '3.6'
     - env: TOXENV=linting,docs,doctesting PYTEST_NO_COVERAGE=1
+      python: '3.7'
 
     - stage: deploy
       python: '3.6'