Update pip to 20.2.4

[edmorley]

Updates pip from 20.1.1 to 20.2.4 for Python 2.7 and Python 3.5+.

Python 3.4 continues to use 19.1.1, the last pip version to support it.

We're not updating to pip 20.3+ yet, since those versions enable the new pip dependency resolver by default, which has compatibility implications, so needs additional UX work first as well as many upstream fixes as possible (see #1109).

Changes:
https://pip.pypa.io/en/latest/news/#v20-2-4
pypa/pip@20.1.1...20.2.4

The new pip wheel was uploaded to S3 using:

$ pip download --no-cache pip==20.2.4
Collecting pip==20.2.4
  Downloading pip-20.2.4-py2.py3-none-any.whl (1.5 MB)
Saved ./pip-20.2.4-py2.py3-none-any.whl
Successfully downloaded pip

$ aws s3 sync . s3://heroku-buildpack-python/common/ --exclude "*" --include "*.whl" --dryrun
(dryrun) upload: ./pip-20.2.4-py2.py3-none-any.whl to s3://heroku-buildpack-python/common/pip-20.2.4-py2.py3-none-any.whl

$ aws s3 sync . s3://heroku-buildpack-python/common/ --exclude "*" --include "*.whl"
upload: ./pip-20.2.4-py2.py3-none-any.whl to s3://heroku-buildpack-python/common/pip-20.2.4-py2.py3-none-any.whl

Closes GUS-W-7944197.