manual: specify interpreter in virtualenv shell

Without this virtualenv might try to setup an environment for a
different version of python then the one specified in the expression.

doc/languages-frameworks/python.section.md

@@ -1034,7 +1034,10 @@ Create this `default.nix` file, together with a `requirements.txt` and simply ex
 
 ```nix
 with import <nixpkgs> {};
-with python27Packages;
+
+let
+  pythonPackages = python27Packages;
+in
 
 stdenv.mkDerivation {
   name = "impurePythonEnv";
@@ -1044,9 +1047,8 @@ stdenv.mkDerivation {
   buildInputs = [
     # these packages are required for virtualenv and pip to work:
     #
-    python27Full
-    python27Packages.virtualenv
-    python27Packages.pip
+    pythonPackages.virtualenv
+    pythonPackages.pip
     # the following packages are related to the dependencies of your python
     # project.
     # In this particular example the python modules listed in the
@@ -1059,14 +1061,13 @@ stdenv.mkDerivation {
     libxml2
     libxslt
     libzip
-    stdenv
     zlib
   ];
 
   shellHook = ''
     # set SOURCE_DATE_EPOCH so that we can use python wheels
     SOURCE_DATE_EPOCH=$(date +%s)
-    virtualenv --no-setuptools venv
+    virtualenv --python=${pythonPackages.python.interpreter} --no-setuptools venv
     export PATH=$PWD/venv/bin:$PATH
     pip install -r requirements.txt
   '';