Motivation for this change

Hi, I was developing my python project when I came across the following error message:

$ nix-shell
Sourcing python-catch-conflicts-hook.sh
Sourcing python-remove-bin-bytecode-hook.sh
Sourcing setuptools-build-hook
Using setuptoolsBuildPhase
Using setuptoolsShellHook
Sourcing pip-install-hook
Using pipInstallPhase
Sourcing python-imports-check-hook.sh
Using pythonImportsCheckPhase
Sourcing setuptools-check-hook
Executing setuptoolsShellHook

Usage:   
  /nix/store/k0igqpz995nmxwf6piyd5jjpcc7gcnyc-python3-3.7.4/bin/python3.7 -m pip <command> [options]

no such option: -e
Finished executing setuptoolsShellHook

This PR addresses the issue with 2 changes: In the responsible hook we tried to call

pip -e

As far as I can tell the author of this code wanted to install the package that the user is currently working on as an "editable" into a temporary directory. This is why this patch calls instead

pip install -e

which "makes the error go away".

Why staging?

I submitted this to staging as I guess a change at this low level will trigger a mass rebuild. I am happy to resubmit this targeting another branch if desired.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @