pip no longer supports Python 2.7 and 3.5

[radarhere]

You can see at https://pypi.org/project/pip/ that it 'Requires: Python >=3.6'

Fixes failures seen in #6 - https://travis-ci.com/github/multi-build/docker-images/jobs/480933865#L688

+ python2.7 get-pip.py
Traceback (most recent call last):
  File "get-pip.py", line 24244, in <module>
    main()
  File "get-pip.py", line 199, in main
    bootstrap(tmpdir=tmpdir)
  File "get-pip.py", line 82, in bootstrap
    from pip._internal.cli.main import main as pip_entry_point
  File "/tmp/tmpzHnbCb/pip.zip/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

[mattip]

Thanks @radarhere