'pip install' fails for every package ("Could not find a version that satisfies the requirement")

pip install <package name> is failing for every package for me. This is what I get:

    Could not find a version that satisfies the requirement <package-name
    (from versions: )
    No matching distribution found for <package-name>

I saw similar questions on Stack Overflow, but they don't seem to be fully related to this one.

Also, this post suggests that this might happen if PyPI is down or my IP address is blacklisted. It seems both are not true for my case.

pip shows up-to-date on running pip install --upgrade pip.

Upgrade pip as follows:

    curl https://bootstrap.pypa.io/get-pip.py | python

Note: You may need to use sudo python above if not in a virtual environment.

What's happening:

Python.org sites are stopping support for TLS versions 1.0 and 1.1. This means that Mac OS X version 10.12 (Sierra) or older will not be able to use pip unless they upgrade pip as above.

(Note that upgrading pip via pip install --upgrade pip will also not upgrade it correctly. It is a chicken-and-egg issue)

This thread explains it (thanks to this Twitter post):

Mac users who use pip and PyPI:

If you are running macOS/OS X version 10.12 or older, then you ought to upgrade to the latest pip (9.0.3) to connect to the Python Package Index securely:

>     curl https://bootstrap.pypa.io/get-pip.py | python
>

and we recommend you do that by April 8th.

Pip 9.0.3 supports TLSv1.2 when running under system Python on macOS < 10.13. Official release notes: https://pip.pypa.io/en/stable/news/

Also, the Python status page:

Completed - The rolling brownouts are finished, and TLSv1.0 and TLSv1.1 have been disabled. Apr 11, 15:37 UTC

Update - The rolling brownouts have been upgraded to a blackout, TLSv1.0 and TLSv1.1 will be rejected with a HTTP 403 at all times. Apr 8, 15:49 UTC

Lastly, to avoid other install errors, make sure you also upgrade setuptools after doing the above:

    pip install --upgrade setuptools

From: stackoverflow.com/q/49748063