bump notebook, nbconvert, jupyterhub

- jupyterhub-singleuser==1.3.0
- nbconvert==6.0.7
- notebook==6.2.0

repo2docker/buildpacks/conda/environment.yml

@@ -4,7 +4,7 @@ dependencies:
   - jupyter-offlinenotebook==0.2.1
   - jupyter-resource-usage==0.5.1
   - jupyterlab==3.0.7
-  - jupyterhub-singleuser==1.1.0
-  - nbconvert==5.6.1
-  - notebook==6.1.6
+  - jupyterhub-singleuser==1.3.0
+  - nbconvert==6.0.7
+  - notebook==6.2.0
   - nteract_on_jupyter==2.1.3