try caching built pythons

.travis.yml

@@ -1,92 +1,98 b''
 # http://travis-ci.org/#!/ipython/ipython
 language: python
 
 addons:
     apt:
         packages:
             - graphviz
 
 python:
     - 3.6
     - 3.5
 
 sudo: false
 
 env:
   global:
     - PATH=$TRAVIS_BUILD_DIR/pandoc:$PATH
+    - PYTHON_BUILD_CACHE_PATH=$HOME/.pyenv_cache
 
 group: edge
 
+cache:
+  directories:
+    - $HOME/.pyenv_cache
+
 before_install:
     - 'if [[ $GROUP != js* ]]; then COVERAGE=""; fi'
     - |
       if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
         brew update
         brew upgrade pyenv
         eval "$(pyenv init -)"
         pyenv install "$PYENV_VERSION"
         pyenv global "$PYENV_VERSION"
         pyenv shell "$PYENV_VERSION"
         pyenv rehash
         python --version
         echo $(python --version)
       fi
 
 
 install:
     - pip install pip --upgrade
     - pip install setuptools --upgrade
     - pip install -e file://$PWD#egg=ipython[test] --upgrade
     - pip install trio curio
     - pip install codecov check-manifest --upgrade
 
 script:
     - check-manifest
     - |
       if [[ "$TRAVIS_PYTHON_VERSION" == "nightly" ]]; then
          # on nightly fake parso known the grammar
          cp /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/parso/python/grammar37.txt /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/parso/python/grammar38.txt
       fi
     - cd /tmp && iptest --coverage xml && cd -
     # On the latest Python (on Linux) only, make sure that the docs build.
     - |
       if [[ "$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:
         - { python: "3.6-dev", dist: xenial, sudo: true, os: linux }
         - { python: "3.7", dist: xenial, sudo: true, os: linux }
         - { python: "3.7-dev", dist: xenial, sudo: true, os: linux }
         - { python: "nightly", dist: xenial, sudo: true, os: linux }
         - { os: osx, language: generic, env: TRAVIS_PYTHON_VERSION="3.6" PYENV_VERSION="3.6.7" }
         - { os: osx, language: generic, env: TRAVIS_PYTHON_VERSION="3.7" PYENV_VERSION="3.7.1" }
     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"