Should python3Packages.imutils support opencv4?

[mtrsk]

Describe the bug Looking at the current derivation of imutils it seems opecv's version is set to 3. This makes some code break if you're working on a opencv4 project, since it'll fall back to version 3.

To Reproduce Steps to reproduce the behavior:

1. Add imutils to an existing opencv4 project,

Given the following shell.nix:

{ pkgs ? import <nixpkgs> {} }:

let
  python = pkgs.python37Packages;

  pythonPkgs = with python; [
    opencv4
  ];

  systemPkgs = with pkgs; [
    opencv4
  ];
in
pkgs.mkShell {
  name = "py-env";
  buildInputs = systemPkgs ++ pythonPkgs;
}

Verify that's running openc4:

$ python
Python 3.7.5 (default, Oct 14 2019, 23:08:55) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
>>> cv2.__version__
'4.1.2'

Now add imutils to pythonPkgs:

>>> import cv2
>>> cv2.__version__
'3.4.8'

Expected behavior The correct version of opencv should be picked.

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.1, NixOS, 20.03pre203904.bb1013511e1 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.1`
 - channels(root): `"nixos-20.03pre203904.bb1013511e1"`
 - channels(usul): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: nixos.python37Packages.imutils
# a list of nixos modules affected by the problem
module:

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