Fix get-pip urls for older pypy versions

[TimPansino]

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 https://github.com/pyenv/pyenv/issues/XXXX

Description

On older recipes for PyPy the current get-pip.py is called, when it returns and incompatible script. Older CPython work around this by supplying the appropriate unsupported minor version number in the URL, such as https://bootstrap.pypa.io/pip/2.7/get-pip.py. This change would incorporate this same logic into PyPy installs such as pypy2.7-7.3.12 where the minor version of Python is specified.

Tests

• Adds unittests to pyenv_ext.bats for previously untested and new pinned bootstrap script versions.

[TimPansino]

Let me know if an accompanying issue is required and I can open one.