Sporadic Pip HTTP errors in CI

[jsirois]

This has actually been happening a while (~1 year): https://github.com/pantsbuild/pex/actions/runs/5432519810/jobs/9879610414#step:8:1734

It looks like so:

E       WARNING: Skipping page https://pypi.org/simple/psutil/ because the GET request got Content-Type: Unknown. The only supported Content-Types are application/vnd.pypi.simple.v1+json, application/vnd.pypi.simple.v1+html, and text/html
E       ERROR: Could not find a version that satisfies the requirement psutil==5.9.1 (from versions: none)
E       ERROR: No matching distribution found for psutil==5.9.1

In particular this very much looks like a PyPI issue on the surface due to the claim of Content-Type: Unknown by Pip:

WARNING: Skipping page https://pypi.org/simple/psutil/ because the GET request got Content-Type: Unknown. The only supported Content-Types are application/vnd.pypi.simple.v1+json, application/vnd.pypi.simple.v1+html, and text/html

I happened upon this though just now: pypa/pip#11340 (comment) and that certainly would explain this issue since CI tests many versions of Pip spanning 20.3.4 <-> 23.1.2 currently but Pex uses a single directory for the Pip cache here:
https://github.com/pantsbuild/pex/blob/c2fa8130d3dfb38f19f550a5bb0a3c69b7459f0d/pex/pip/tool.py#L304-L305