update python to use virtualenv

python_environment/default.nix

@@ -3,16 +3,16 @@ with import <nixpkgs> {};
 stdenv.mkDerivation {
   name = "myPythonEnv";
   buildInputs = with pkgs; [
-    python36Full
-    python36Packages.virtualenv
-    python36Packages.pip
+    python37Full
+    python37Packages.virtualenv
   ];
   src = null;
   shellHook = ''
-    # set SOURCE_DATE_EPOCH so that we can use python wheels
-    SOURCE_DATE_EPOCH=$(date +%s)
-    virtualenv --no-setuptools venv
-    export PATH=$PWD/venv/bin:$PATH
+    if [ ! -d .venv ]; then
+      python -m venv .venv
+    fi
+    source .venv/bin/activate
+    pip install --upgrade pip
     if [ -s requirements.txt ]; then
       pip install -r requirements.txt
     fi