Allow pip install -e . to work

[Gerardwx]

When I tried pip install -e . I received the error message
ERROR: File "setup.py" not found. Directory cannot be installed in editable mode: /git/twine
(A "pyproject.toml" file was found, but editable mode currently requires a setup.py based build.)

A stackoverflow post suggested creating this file, which seems to work.

[bhrutledge]

Thanks for flagging this. We removed setup.py recently, in #757, because newer versions of pip don't require it.

What's the output of this command?

python3 -m pip --version

Can you try upgrading?

python3 -m pip install -U pip

This is a good thing to do regularly, and whenever you create a new virtual environment.

That said, I'm guessing other folks will encounter this issue, so it might be worth adding the recommendation to upgrade pip in our contributing guide.

[Gerardwx]

Oh, the default Ubuntu18.04 pip is ancient. The pip upgrade works.