Revert "Use buildPythonPackage development mode instead of mkShell"

This reverts commit aafa11d.

shell.nix

@@ -13,8 +13,8 @@ with import (builtins.fetchTarball {
 
 let python = python37; in
 
-python.pkgs.buildPythonPackage {
-  name = "invisible-cities";
+pkgs.mkShell {
+  pname = "invisible-cities";
   src = ./.;
   buildInputs = with python.pkgs; [
     cython
@@ -31,6 +31,9 @@ python.pkgs.buildPythonPackage {
     scipy
     sphinx
     tornado
+    pytest
+    flaky
+    hypothesis
     #pyxest-xdist
     #pytest-instafail
     python-language-server
@@ -39,15 +42,14 @@ python.pkgs.buildPythonPackage {
     importmagic
   ];
 
-  checkInputs = with python.pkgs; [ pytest flaky hypothesis ];
-
   shellHook = ''
     export ICTDIR=`pwd`
     export ICDIR=$ICTDIR/invisible_cities/
     export PYTHONPATH=$ICTDIR:$PYTHONPATH
     export NBDIR=`pwd`
     export IC_NOTEBOOK_DIR=$NBDIR/invisible_cities/
     export PATH=$ICTDIR/bin:$PATH
+    python setup.py build_ext --inplace
   '';
 
 }