Use --no-use-pep517 flag for editable mode with pyproject.toml

[matthewfeickert]

Description

The reasons for this are probably best given in pip Issue 6434 and PR 6442. In short, though, with pip v19.1 enforces PEP 517 which does not allow for editable mode installations

Editable installs

This PEP originally specified another hook, install_editable, to do an editable install (as with pip install -e). It was removed due to the complexity of the topic, but may be specified in a later PEP.

As pyhf has a pyproject.toml file for Black this breaks the setup as there is no PEP yet to specify editable installations for developers with pyproject.toml. pip PR 6442 fixes this with --no-use-pep517 for the time being. This PR adopts --no-use-pep517 for all installations.

This will also need to get addressed later in Issue #399

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use --no-use-pep517 flag for editable mode during installation and update docs to reflect this
   - https://github.com/pypa/pip/issues/6434
   - https://github.com/pypa/pip/pull/6442
* Add .eggs to Black exclude list

[coveralls]

Coverage remained the same at 97.559% when pulling ea76f0d on fix/pip-v19.1-compliant-no-use-pep517 into 44e0701 on master.