python2Packages: keep separate (bootstrapped-)pip

Currently there is only one line difference regarding bootstrapped-pip,
but this will change in the future with the whole Python world moving
towards PEP 517.

pkgs/development/python-modules/pip/20.nix

@@ -0,0 +1,44 @@
+{ lib
+, python
+, buildPythonPackage
+, bootstrapped-pip
+, fetchFromGitHub
+, mock
+, scripttest
+, virtualenv
+, pretend
+, pytest
+, setuptools
+, wheel
+}:
+
+buildPythonPackage rec {
+  pname = "pip";
+  version = "20.3.4";
+  format = "other";
+
+  src = fetchFromGitHub {
+    owner = "pypa";
+    repo = pname;
+    rev = version;
+    sha256 = "0hkhs9yc1cjdj1gn9wkycd3sy65c05q8k8rhqgsm5jbpksfssiwn";
+    name = "${pname}-${version}-source";
+  };
+
+  nativeBuildInputs = [ bootstrapped-pip ];
+
+  # pip detects that we already have bootstrapped_pip "installed", so we need
+  # to force it a little.
+  pipInstallFlags = [ "--ignore-installed" ];
+
+  checkInputs = [ mock scripttest virtualenv pretend pytest ];
+  # Pip wants pytest, but tests are not distributed
+  doCheck = false;
+
+  meta = {
+    description = "The PyPA recommended tool for installing Python packages";
+    license = with lib.licenses; [ mit ];
+    homepage = "https://pip.pypa.io/";
+    priority = 10;
+  };
+}