Update pip to 20.3.4/21.3.1 (new dependency resolver)

[edmorley]

Update pip from 20.2.4 to:

• 20.3.4 for Python 2.7 and 3.5
• 21.3.1 for Python 3.6+

Of note Pip 20.3+ includes the new dependency resolver (only enabled by default when using Python 3+). This new dependency resolver is more strict, see:
https://pip.pypa.io/en/stable/user_guide/#changes-to-the-pip-dependency-resolver-in-20-3-2020
https://pip.pypa.io/en/stable/topics/dependency-resolution/

Release notes:
https://pip.pypa.io/en/stable/news/#v21-3-1

Changelog:
pypa/pip@20.2.4...21.3.1

The new versions of pip have been synced to S3 using:

$ pip download --no-cache pip==20.3.4
...
Saved ./pip-20.3.4-py2.py3-none-any.whl
Successfully downloaded pip
$ pip download --no-cache pip==21.3.1
Collecting pip==21.3.1
...
Saved ./pip-21.3.1-py3-none-any.whl
Successfully downloaded pip
$ aws s3 sync . s3://heroku-buildpack-python/common/ --exclude "*" --include "*.whl" --dryrun
(dryrun) upload: ./pip-20.3.4-py2.py3-none-any.whl to s3://heroku-buildpack-python/common/pip-20.3.4-py2.py3-none-any.whl
(dryrun) upload: ./pip-21.3.1-py3-none-any.whl to s3://heroku-buildpack-python/common/pip-21.3.1-py3-none-any.whl
$ aws s3 sync . s3://heroku-buildpack-python/common/ --exclude "*" --include "*.whl"
upload: ./pip-20.3.4-py2.py3-none-any.whl to s3://heroku-buildpack-python/common/pip-20.3.4-py2.py3-none-any.whl
upload: ./pip-21.3.1-py3-none-any.whl to s3://heroku-buildpack-python/common/pip-21.3.1-py3-none-any.whl

Closes #1109.
GUS-W-8493316.

[edmorley]

Example of a new error shown when conflicting (invalid) dependency combinations are attempted to be installed:

$ pip install requests==2.26.0 'urllib3<1.21'
DEPRECATION: Configuring installation scheme with distutils config files is deprecated and will no longer work in the near future. If you are using a Homebrew or Linuxbrew Python, please see discussion at https://github.com/Homebrew/homebrew-core/issues/76621
Collecting requests==2.26.0
  Downloading requests-2.26.0-py2.py3-none-any.whl (62 kB)
     |████████████████████████████████| 62 kB 2.2 MB/s
Collecting urllib3<1.21
  Downloading urllib3-1.20-py2.py3-none-any.whl (111 kB)
     |████████████████████████████████| 111 kB 29.1 MB/s
Requirement already satisfied: idna<4,>=2.5 in /usr/local/lib/python3.9/site-packages (from requests==2.26.0) (2.10)
Collecting charset-normalizer~=2.0.0
  Downloading charset_normalizer-2.0.7-py3-none-any.whl (38 kB)
Requirement already satisfied: certifi>=2017.4.17 in /usr/local/lib/python3.9/site-packages (from requests==2.26.0) (2020.12.5)
ERROR: Cannot install requests==2.26.0 and urllib3<1.21 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested urllib3<1.21
    requests 2.26.0 depends on urllib3<1.27 and >=1.21.1

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies

I'll cover these in the changelog entry (the pip docs linked from the PR description explains how to resolve these).

[edmorley]

https://devcenter.heroku.com/changelog-items/2288

[edmorley]

For apps that need a temporary workaround until you have time to fix the app's dependencies, you can pin to the previous version of the buildpack (v201), using the approach described here:
https://devcenter.heroku.com/articles/buildpacks#buildpack-references