Stop testing on -dev, they are broken on travis-ci

pip is missing on recent builds; and remove duplicate 3.7 testing if we
drop -dev suffix

.travis.yml

@@ -1,122 +1,118 b''
 # http://travis-ci.org/#!/ipython/ipython
 language: python
 os: linux
 
 addons:
   apt:
     packages:
       - graphviz
 
 python:
     - 3.6
 
 sudo: false
 
 env:
   global:
     - PATH=$TRAVIS_BUILD_DIR/pandoc:$PATH
 
 group: edge
 
 before_install:
   - |
     # install Python on macOS
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       env | sort
       if ! which python$TRAVIS_PYTHON_VERSION; then
         HOMEBREW_NO_AUTO_UPDATE=1 brew tap carreau/homebrew-python-frameworks
         HOMEBREW_NO_AUTO_UPDATE=1 brew cask install python-framework-${TRAVIS_PYTHON_VERSION/./}
       fi
       python3 -m pip install virtualenv
       python3 -m virtualenv -p $(which python$TRAVIS_PYTHON_VERSION) ~/travis-env
       source ~/travis-env/bin/activate
     fi
   - python --version
 
 install:
   - pip install pip --upgrade
   - pip install setuptools --upgrade
   - pip install -e file://$PWD#egg=ipython[test] --upgrade
   - pip install trio curio --upgrade --upgrade-strategy eager
   - pip install pytest matplotlib
   - pip install codecov check-manifest --upgrade
 
 script:
   - check-manifest
   - |
     if [[ "$TRAVIS_PYTHON_VERSION" == "nightly" ]]; then
        # on nightly fake parso known the grammar
        cp /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/parso/python/grammar37.txt /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/parso/python/grammar38.txt
     fi
   - cd /tmp && iptest --coverage xml && cd -
   - pytest IPython
   # On the latest Python (on Linux) only, make sure that the docs build.
   - |
     if [[ "$TRAVIS_PYTHON_VERSION" == "3.7" ]] && [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
       pip install -r docs/requirements.txt
       python tools/fixup_whats_new_pr.py
       make -C docs/ html SPHINXOPTS="-W"
     fi
 
 after_success:
   - cp /tmp/ipy_coverage.xml ./
   - cp /tmp/.coverage ./
   - codecov
 
 matrix:
   include:
     - arch: amd64
       python: "3.7"
       dist: xenial
       sudo: true
     - arch: arm64
       python: "3.7"
       dist: xenial
       env: ARM64=True
       sudo: true
     - arch: amd64
-      python: "3.8-dev"
-      dist: xenial
-      sudo: true
-    - arch: amd64
-      python: "3.7-dev"
+      python: "3.8"
       dist: xenial
       sudo: true
     - arch: amd64
       python: "nightly"
       dist: xenial
       sudo: true
     - arch: arm64
       python: "nightly"
       dist: bionic
       env: ARM64=True
       sudo: true
     - os: osx
       language: generic
       python: 3.6
       env: TRAVIS_PYTHON_VERSION=3.6
     - os: osx
       language: generic
       python: 3.7
       env: TRAVIS_PYTHON_VERSION=3.7
   allow_failures:
     - python: nightly
 
 before_deploy:
   - rm -rf dist/
   - python setup.py sdist
   - python setup.py bdist_wheel
 
 deploy:
   provider: releases
   api_key:
     secure: Y/Ae9tYs5aoBU8bDjN2YrwGG6tCbezj/h3Lcmtx8HQavSbBgXnhnZVRb2snOKD7auqnqjfT/7QMm4ZyKvaOEgyggGktKqEKYHC8KOZ7yp8I5/UMDtk6j9TnXpSqqBxPiud4MDV76SfRYEQiaDoG4tGGvSfPJ9KcNjKrNvSyyxns=
   file: dist/*
   file_glob: true
   skip_cleanup: true
   on:
     repo: ipython/ipython
     all_branches: true  # Backports are released from e.g. 5.x branch
     tags: true
     python: 3.6  # Any version should work, but we only need one
     condition: $TRAVIS_OS_NAME = "linux"