Add postBuild file and move Binder config in subdir

Author: martinRenou

environment.yml renamed to .binder/environment.yml

@@ -2,6 +2,8 @@ name: ipympl
 channels:
   - conda-forge
 dependencies:
+  - yarn
   - numpy
   - jupyterlab>=3
   - matplotlib-base>=2.2.2
+  - jupyter-packaging=0.7

.binder/postBuild

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+python -m pip install -vvv -e .
+jupyter labextension develop . --overwrite