You need to make sure you have the right version of pip
for Python 2.7:
pip install --upgrade "pip < 21.0"
That versions will be working for some time. Sooner or later they fail because PyPI host changes SSL configuration to one that Python 2.7 doesn't handle. But I'm sure we have a few years before that.
A few additional version limits for important tools to retain compatibility with Python 2.7:
pip install --upgrade "setuptools < 45" "Sphinx < 2.0" "pytest < 5.0"
If you use other tools follow their announcements thoroughly to catch them stopping supporting 2.7.
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