Expose session start file in __file__.

Following jupyter-server/jupyter_server#1100, this expose the value of
JUPYTER_SESSION_NAME environment variable as `__file__`. In _most_
cases, this should reflect the notebook file that correspond to current
kernel.

It is not set when the env variable is not set. And it represent the
name of file at the time the session was created, it will not reflect
renaming of the file.

We could support renaming, but that would require the frontend to send
the name of the current file; this would be helpful when we have
multiple frontends/documents connected to the same kernel, but this
would be out of scope for this PR and a longer discussion to have.

ipykernel/ipkernel.py

@@ -3,6 +3,7 @@
 import asyncio
 import builtins
 import getpass
+import os
 import signal
 import sys
 import threading
@@ -126,6 +127,11 @@ def __init__(self, **kwargs):
             compiler_class=XCachingCompiler,
         )
         self.shell.displayhook.session = self.session
+
+        jupyter_session_name = os.environ.get('JPY_SESSION_NAME')
+        if jupyter_session_name:
+            self.shell.user_ns['__file__'] = jupyter_session_name
+
         self.shell.displayhook.pub_socket = self.iopub_socket
         self.shell.displayhook.topic = self._topic("execute_result")
         self.shell.display_pub.session = self.session