pip 20.3 release breaks source install

[sammdot]

Summary

Building Plover from source causes conflicts in pip's package resolution algorithm. See #1176 for an example of what the error looks like -- it seems pip takes the duplicate installation of local Plover and PyPI Plover as a conflict rather than automatically resolving to the local version (as was the behavior in older versions of pip).

pip documentation says this about the new package resolver:

• Providing an editable requirement (-e .) does not cause pip to ignore version specifiers or constraints (see Relaxing / Ignoring constraints during dependency resolution pypa/pip#8076), and if you have a conflict between a pinned requirement and a local directory then pip will indicate that it cannot find a version satisfying both (see -e . does not take precedence over pin in requirements file pypa/pip#8307)

While Plover's requirements.txt specifies pip==20.0.2, it seems the build process forces an upgrade to 20.3, which is the first version that introduces the new resolution behavior.

Reproducing

• Clone Plover repository at 4.0.0.dev8+235 (d20167f)

• Run the typical development build commands:

  python3 -m pip install -r requirements.txt
  python3 -m pip install --user -e . -r requirements_plugins.txt
  

• Build a macOS app bundle:

  python3 ./setup.py bdist_app
  

The build fails with the following error message:

ERROR: Cannot install -r requirements_plugins.txt (line 19), plover 4.0.0.dev8 (from ~/plover/dist/plover-4.0.0.dev8-py3-none-any.whl) and plover[gui-qt]==4.0.0.dev8 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested plover 4.0.0.dev8 (from ~/plover/dist/plover-4.0.0.dev8-py3-none-any.whl)
    plover-treal 1.0.1 depends on plover>=4.0.0.dev0
    plover[gui-qt] 4.0.0.dev8 depends on plover 4.0.0.dev8 (from https://files.pythonhosted.org/packages/96/09/31e7455012feb2022bb5404308c5c6026ab192840927a46471c95903c31e/plover-4.0.0.dev8-py3-none-any.whl#sha256=c9f42573acd3729b6750d3c7644f81086512ae6ed831dea1440d1f113daa4225 (from https://pypi.org/simple/plover/))

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies

Plover Version

4.0.0.dev8+235 (d20167f)

System

macOS 11.0 Big Sur, Python 3.8.5