Fix issue with pip 20.0 breaking on install

[ericholscher]

For those finding this on GitHub, this is the step to reproduce thanks to @agjohnson:

rm -rf tmp/a
python3.7 -mvirtualenv --system-site-packages --no-download /tmp/a
/tmp/a/bin/python -m pip install --upgrade --cache-dir /tmp/a.cache pip
python3.7 -mvirtualenv --system-site-packages --no-download /tmp/a
/tmp/a/bin/python -m pip install --upgrade --cache-dir /tmp/a.cache pip

Example running in our docker container:

-> docker run --rm -it readthedocs/build:5.0 /bin/bash
docs@ae7bd8db6007:/$ set -x
docs@ae7bd8db6007:/$
docs@ae7bd8db6007:/$ rm -rf tmp/a
+ rm -rf tmp/a
docs@ae7bd8db6007:/$ python3.7 -mvirtualenv --system-site-packages --no-download /tmp/a
+ python3.7 -mvirtualenv --system-site-packages --no-download /tmp/a
Using base prefix '/home/docs/.pyenv/versions/3.7.3'
New python executable in /tmp/a/bin/python3.7
Also creating executable in /tmp/a/bin/python
Installing setuptools, pip, wheel...
done.
docs@ae7bd8db6007:/$ /tmp/a/bin/python -m pip install --upgrade --cache-dir /tmp/a.cache pip
+ /tmp/a/bin/python -m pip install --upgrade --cache-dir /tmp/a.cache pip
Collecting pip
  Downloading https://files.pythonhosted.org/packages/54/0c/d01aa759fdc501a58f431eb594a17495f15b88da142ce14b5845662c13f3/pip-20.0.2-py2.py3-none-any.whl (1.4MB)
     |████████████████████████████████| 1.4MB 908kB/s
Installing collected packages: pip
  Found existing installation: pip 19.1.1
    Uninstalling pip-19.1.1:
      Successfully uninstalled pip-19.1.1
Successfully installed pip-20.0.2
docs@ae7bd8db6007:/$ python3.7 -mvirtualenv --system-site-packages --no-download /tmp/a
+ python3.7 -mvirtualenv --system-site-packages --no-download /tmp/a
Using base prefix '/home/docs/.pyenv/versions/3.7.3'
New python executable in /tmp/a/bin/python3.7
Not overwriting existing python script /tmp/a/bin/python (you must use /tmp/a/bin/python3.7)
Installing setuptools, pip, wheel...
done.
docs@ae7bd8db6007:/$ /tmp/a/bin/python -m pip install --upgrade --cache-dir /tmp/a.cache pip
+ /tmp/a/bin/python -m pip install --upgrade --cache-dir /tmp/a.cache pip
Traceback (most recent call last):
  File "/home/docs/.pyenv/versions/3.7.3/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/docs/.pyenv/versions/3.7.3/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/tmp/a/lib/python3.7/site-packages/pip/__main__.py", line 16, in <module>
    from pip._internal import main as _main  # isort:skip # noqa
  File "/tmp/a/lib/python3.7/site-packages/pip/_internal/__init__.py", line 40, in <module>
    from pip._internal.cli.autocompletion import autocomplete
  File "/tmp/a/lib/python3.7/site-packages/pip/_internal/cli/autocompletion.py", line 8, in <module>
    from pip._internal.cli.main_parser import create_main_parser
  File "/tmp/a/lib/python3.7/site-packages/pip/_internal/cli/main_parser.py", line 12, in <module>
    from pip._internal.commands import (
  File "/tmp/a/lib/python3.7/site-packages/pip/_internal/commands/__init__.py", line 6, in <module>
    from pip._internal.commands.completion import CompletionCommand
  File "/tmp/a/lib/python3.7/site-packages/pip/_internal/commands/completion.py", line 6, in <module>
    from pip._internal.cli.base_command import Command
  File "/tmp/a/lib/python3.7/site-packages/pip/_internal/cli/base_command.py", line 25, in <module>
    from pip._internal.index import PackageFinder
ImportError: cannot import name 'PackageFinder' from 'pip._internal.index' (/tmp/a/lib/python3.7/site-packages/pip/_internal/index/__init__.py)

[agjohnson]

👍

[LarryOltmanns]

Removed all Python installations, including down loader. Then installed Python 3.7.4 64 bit includes installer. Ran commands - to use python it works. Now with dwave-ocean

[LarryOltmanns]

py -m pip --version
pip 9.0.1 from c:\python36\lib\site-packages (Python 3.6.1)
You can make sure that pip is up-to-date by running:

py -m pip install --upgrade pip

[LarryOltmanns]

Windows cmd.exe
py -m pip install --upgrade pip
(this removes the 19.2.3 and installs 20.0.2