pytest: fixed package requirements for latest setuptools which breaks backward compat.

pkgs/patches/pytest/setuptools.patch

@@ -0,0 +1,12 @@
+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={

pkgs/python-packages-overrides.nix

@@ -274,6 +274,12 @@ self: super: {
     ];
   });
 
+  "pytest" = super."pytest".override (attrs: {
+    patches = [
+      ./patches/pytest/setuptools.patch
+    ];
+  });
+
   # Avoid that base packages screw up the build process
   inherit (basePythonPackages)
     setuptools;