Defer webbrowser import (#1095)

Fixes #1094

Author: blink1073

jupyter_server/serverapp.py

@@ -23,7 +23,6 @@
 import time
 import urllib
 import warnings
-import webbrowser
 from base64 import encodebytes
 
 from jupyter_client.kernelspec import KernelSpecManager
@@ -2748,6 +2747,10 @@ def _prepare_browser_open(self):
         return assembled_url, open_file
 
     def launch_browser(self):
+        # Deferred import for environments that do not have
+        # the webbrowser module.
+        import webbrowser
+
         try:
             browser = webbrowser.get(self.browser or None)
         except webbrowser.Error as e: