Update setup.py to handle source installs

Without this Binder errors out because of missing jupyter-packaging,
see jupyterlab/extension-cookiecutter-ts#148

Author: krassowski

setup.py

@@ -2,9 +2,9 @@
 jupyterlab_execute_time setup.
 """
 import json
+import sys
 from pathlib import Path
 import setuptools
-from jupyter_packaging import wrap_installers, npm_builder, get_data_files
 
 HERE = Path(__file__).parent.resolve()
 
@@ -65,13 +65,24 @@
 )
 
 
-post_develop = npm_builder(
-    build_cmd="install:extension", source_dir="src", build_dir=lab_path, npm="jlpm"
-)
-setup_args["cmdclass"] = wrap_installers(
-    post_develop=post_develop, ensured_targets=ensured_targets
-)
-setup_args["data_files"] = get_data_files(data_files_spec)
+try:
+    from jupyter_packaging import (
+        wrap_installers,
+        npm_builder,
+        get_data_files
+    )
+    post_develop = npm_builder(
+        build_cmd="install:extension", source_dir="src", build_dir=lab_path, npm="jlpm"
+    )
+    setup_args["cmdclass"] = wrap_installers(post_develop=post_develop, ensured_targets=ensured_targets)
+    setup_args["data_files"] = get_data_files(data_files_spec)
+except ImportError as e:
+    import logging
+    logging.basicConfig(format="%(levelname)s: %(message)s")
+    logging.warning("Build tool `jupyter-packaging` is missing. Install it with pip or conda.")
+    if not ("--name" in sys.argv or "--version" in sys.argv):
+        raise e
+
 
 if __name__ == "__main__":
     setuptools.setup(**setup_args)