[issue38488] Update bundled pip to 19.3

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

The change broke multiple buildbots. Since nobody is available to investigate the two regressions (install buildbots and test_venv), I reverted the change following this policy:
https://pythondev.readthedocs.io/ci.html#revert-on-fail

Buildbot failures were reported directly the on PR, see:
https://github.com/python/cpython/pull/16782#issuecomment-634640005

The revert is an opportunity to have more time to investigate the issue and write a proper fix, rather than working on urgency.

commit 4fd4963ccce5c12f742303dab6e43818b1133c7e (HEAD -> master, upstream/master)
Author: Victor Stinner <vstinner at python.org>
Date:   Thu May 28 15:57:49 2020 +0200

    Revert "Upgrade bundled versions of pip & setuptools (#16782)" (GH-20484)
    
    This reverts commit feb0846c3a28b05b4cfbc6ab34c764957f3eff55.

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