Use GNU Readline 8.0

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Make sure you have checked all steps below.

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • This is a fix to its core build feature
• x ] 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.
  • The feature isn't derived from the upstream
• My PR addresses the following pyenv issue (if any)
  • n/a

Description

• Here are some details about my PR

Update GNU Readline version to be installed when brew's package is not available.

Tests

• My PR adds the following unit tests (if any)
  • n/a