Update pip to 20.1.1 (#1030)

Updates pip from 20.0.2 to 20.1.1 for Python 2.7 and Python 3.5+:
https://pip.pypa.io/en/stable/news/#id40

The version used for Python 3.4 remains unchanged at 19.1.1, since it's
the last version of pip that supports it.

Pip has been updated to 20.1.1 rather than the recently released 20.2,
since the latter has a few regressions and even though these will be
fixed shortly in 20.2.1, we should let the changes soak for longer
before picking them up.

The `PIP_NO_PYTHON_VERSION_WARNING` environment variable has been set
(equivalent to passing `--no-python-version-warning`) to prevent the
Python 2.7 EOL warnings added in pip 20.1 from spamming the build log:
https://github.com/pypa/pip/blob/20.1.1/src/pip/_internal/cli/base_command.py#L139-L154

This was set via environment variable rather than CLI flag, since:
* otherwise we'd have to pass it to every pip invocation
* older pip (such as the 19.1.1 used by Python 3.4) doesn't support this
  option and would error out due to an unknown CLI flag being passed,
  unless we added conditional flags throughout.

The new pip wheel was uploaded to S3 using:

```
$ pip download --no-cache pip==20.1.1
Collecting pip==20.1.1
  Downloading pip-20.1.1-py2.py3-none-any.whl (1.5 MB)
  Saved ./pip-20.1.1-py2.py3-none-any.whl
Successfully downloaded pip

$ aws s3 sync . s3://lang-python/common/ --exclude "*" --include "*.whl" --acl public-read --dryrun
(dryrun) upload: ./pip-20.1.1-py2.py3-none-any.whl to s3://lang-python/common/pip-20.1.1-py2.py3-none-any.whl

$ aws s3 sync . s3://lang-python/common/ --exclude "*" --include "*.whl" --acl public-read
upload: ./pip-20.1.1-py2.py3-none-any.whl to s3://lang-python/common/pip-20.1.1-py2.py3-none-any.whl
```

Fixes #1005.
@W-7659489@

bin/steps/python

@@ -137,7 +137,7 @@ fi
 
 set -e
 
-PIP_VERSION='20.0.2'
+PIP_VERSION='20.1.1'
 # Must use setuptools <47.2.0 until we fix:
 # https://github.com/heroku/heroku-buildpack-python/issues/1006
 SETUPTOOLS_VERSION='47.1.1'