build: fixed setuptools

pkgs/patches/pytest/setuptools.patch

@@ -0,0 +1,12 b''
+diff -rup pytest-4.6.5-orig/setup.py pytest-4.6.5/setup.py
+--- pytest-4.6.5-orig/setup.py	2018-04-10 10:23:04.000000000 +0200
++++  pytest-4.6.5/setup.py	2018-04-10 10:23:34.000000000 +0200
+@@ -24,7 +24,7 @@ INSTALL_REQUIRES = [
+ def main():
+     setup(
+         use_scm_version={"write_to": "src/_pytest/_version.py"},
+-        setup_requires=["setuptools-scm", "setuptools>=40.0"],
++        setup_requires=["setuptools-scm", "setuptools<=42.0"],
+         package_dir={"": "src"},
+         # fmt: off
+         extras_require={
  No newline at end of file

pkgs/python-packages-overrides.nix

@@ -1,72 +1,78 b''
 # Overrides for the generated python-packages.nix
 #
 # This function is intended to be used as an extension to the generated file
 # python-packages.nix. The main objective is to add needed dependencies of C
 # libraries and tweak the build instructions where needed.
 
 { pkgs
 , basePythonPackages
 }:
 
 let
   sed = "sed -i";
 
 in
 
 self: super: {
 
   "cffi" = super."cffi".override (attrs: {
     buildInputs = [
       pkgs.libffi
     ];
   });
 
   "gevent" = super."gevent".override (attrs: {
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       # NOTE: (marcink) odd requirements from gevent aren't set properly,
       # thus we need to inject psutil manually
       self."psutil"
     ];
   });
 
   "hgsubversion" = super."hgsubversion".override (attrs: {
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       pkgs.sqlite
       #basePythonPackages.sqlite3
       self.mercurial
     ];
   });
 
   "subvertpy" = super."subvertpy".override (attrs: {
     SVN_PREFIX = "${pkgs.subversion.dev}";
     propagatedBuildInputs = [
       pkgs.apr.dev
       pkgs.aprutil
       pkgs.subversion
     ];
   });
 
   "mercurial" = super."mercurial".override (attrs: {
     propagatedBuildInputs = [
       # self.python.modules.curses
     ];
   });
 
   "dulwich" = super."dulwich".override (attrs: {
     patches = [
       ./patches/dulwich/handle-dir-refs.patch
     ];
   });
 
   "pygit2" = super."pygit2".override (attrs: {
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       pkgs.libffi
       pkgs.libgit2rc
     ];
   });
 
+  "pytest" = super."pytest".override (attrs: {
+    patches = [
+      ./patches/pytest/setuptools.patch
+    ];
+  });
+
   # Avoid that base packages screw up the build process
   inherit (basePythonPackages)
     setuptools;
 
 }