check available python versions via pyenv

Author: snowman2

.travis.yml

@@ -26,7 +26,7 @@ matrix:
     - language: cpp
       os: osx
       env:
-        - PYTHON=3.7.9
+        - PYTHON=3.7.8
     - python: 3.7.9
     - python: 3.7.9
       env:
@@ -63,6 +63,7 @@ before_install:
       # virtualenv doesn't work without pyenv knowledge. venv in Python 3.3
       # doesn't provide Pip by default. So, use `pyenv-virtualenv <https://github.com/yyuu/pyenv-virtualenv/blob/master/README.md>`_.
       brew install pyenv-virtualenv
+      pyenv install --list | grep "3.7"
       pyenv install $PYTHON
       # I would expect something like ``pyenv init; pyenv local $PYTHON`` or
       # ``pyenv shell $PYTHON`` would work, but ``pyenv init`` doesn't seem to