cpython: fix permissions on venv activation scripts

Previously these ended up without u+w permissions which meant they could
not be regenerated, which was hugely annoying when these venvs were for
example created and recreated in a nix-shell.

pkgs/development/interpreters/python/cpython/default.nix

@@ -190,6 +190,10 @@ in with passthru; stdenv.mkDerivation {
     # (since it will do a futile invocation of gcc (!) to find
     # libuuid, slowing down program startup a lot).
     (./. + "/${sourceVersion.major}.${sourceVersion.minor}/no-ldconfig.patch")
+    # Make sure that the virtualenv activation scripts are
+    # owner-writable, so venvs can be recreated without permission
+    # errors.
+    ./virtualenv-permissions.patch
   ] ++ optionals mimetypesSupport [
     # Make the mimetypes module refer to the right file
     ./mimetypes.patch

pkgs/development/interpreters/python/cpython/virtualenv-permissions.patch

@@ -0,0 +1,13 @@
+diff --git a/Lib/venv/__init__.py b/Lib/venv/__init__.py
+index caa7285..ad666ac 100644
+--- a/Lib/venv/__init__.py
++++ b/Lib/venv/__init__.py
+@@ -379,7 +379,7 @@ class EnvBuilder:
+                 if data is not None:
+                     with open(dstfile, 'wb') as f:
+                         f.write(data)
+-                    shutil.copymode(srcfile, dstfile)
++                    os.chmod(dstfile, 0o644)
+ 
+ 
+ def create(env_dir, system_site_packages=False, clear=False,