Allow multiple versions for pyenv-install

[rockandska]

Make sure you have checked all steps below.

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
• My PR addresses the following pyenv issue (if any)
  • Closes pyenv install without args fails with multiple versions in .python-version #919

Description

As stated in #919, many of the users (as me) expect to have pyenv to install all versions specified in .python-version and have pyenv install works the same way as pyenv local works

Tests

• My PR adds the following unit tests (if any)
  • install proper multi versions
  • install resolves a prefix with multi versions
  • install pyenv local multi versions by default
  • multi arguments for pyenv-install
• My PR remove the following unit tests (if any)
  • too many arguments for pyenv-install
    • replaced by multi arguments for pyenv-install since user could specify multi versions now

[colindean]

Thank you!

[rockandska]

Just seen that it was added too in actions/setup-python#567
Perfect timing ^^