Lay foundation to pass notebook names to kernel at startup.

This has been a controversial topic from some time:

jupyter/notebook#1000
https://forums.databricks.com/questions/21390/is-there-any-way-to-get-the-current-notebook-name.html
https://stackoverflow.com/questions/12544056/how-do-i-get-the-current-ipython-jupyter-notebook-name
https://ask.sagemath.org/question/36873/access-notebook-filename-from-jupyter-with-sagemath-kernel/

This is also sometime critical to linter, and tab completion to know
current name.

Of course current answer is that the question is ill-defined,
there might not be a file associated with the current kernel, there
might be multiple files, files might not be on the same system, it could
change through the execution and many other gotchas.

This suggest to add an JPY_ASSOCIATED_FILE env variable which is not
too visible, but give an escape hatch which should mostly be correct
unless the notebook is renamed or kernel attached to a new one.

Do do so this handles the new associated_file parameters in a few
function of the kernel manager. On jupyter_server this one line change
make the notebook name available using typical local installs:

    --- a/jupyter_server/services/sessions/sessionmanager.py
    +++ b/jupyter_server/services/sessions/sessionmanager.py
    @@ -96,7 +96,12 @@ class SessionManager(LoggingConfigurable):
             """Start a new kernel for a given session."""
             # allow contents manager to specify kernels cwd
             kernel_path = self.contents_manager.get_kernel_path(path=path)
    -        kernel_id = await self.kernel_manager.start_kernel(path=kernel_path, kernel_name=kernel_name)
    +
    +        kernel_id = await self.kernel_manager.start_kernel(
    +            path=kernel_path, kernel_name=kernel_name, associated_file=name
    +        )
             return kernel_id

Of course only launchers that will pass forward this value will allow
the env variable to be set.

I'm thinking that various kernels may use this and expose it in
different ways. like __notebook_name__ if it ends with `.ipynb` in
ipykernel.

Author: Carreau

jupyter_client/manager.py

@@ -35,6 +35,15 @@
 from jupyter_client import launch_kernel
 
 
+# Name of the env variable that contains the name of the current file associated
+# with the kernel we are launching.
+# this is useful in notebook context to find which notebook we are working with
+# though we might not be working with a notebook, we could be working with a
+# markdown file, or python file.
+# as with other Jupyter Related Env variable with use the JPY prefix.
+ASSOCIATED_FILE = 'JPY_ASSOCIATED_FILE'
+
+
 class _ShutdownStatus(Enum):
     """
 
@@ -298,6 +307,9 @@ def pre_start_kernel(self, **kw) -> t.Tuple[t.List[str], t.Dict[str, t.Any]]:
         # Don't allow PYTHONEXECUTABLE to be passed to kernel process.
         # If set, it can bork all the things.
         env.pop("PYTHONEXECUTABLE", None)
+        associated_file = kw.pop('associated_file', None)
+        if (env.get(ASSOCIATED_FILE, None) is None) and (associated_file is not None):
+            env[ASSOCIATED_FILE] = associated_file
 
         # Environment variables from kernel spec are added to os.environ.
         assert self.kernel_spec is not None

jupyter_client/multikernelmanager.py

@@ -180,6 +180,9 @@ def pre_start_kernel(
         constructor_kwargs = {}
         if self.kernel_spec_manager:
             constructor_kwargs["kernel_spec_manager"] = self.kernel_spec_manager
+
+        if 'associated_file' in kwargs:
+            constructor_kwargs['associated_file'] = kwargs.pop('associated_file')
         km = self.kernel_manager_factory(
             connection_file=os.path.join(self.connection_dir, "kernel-%s.json" % kernel_id),
             parent=self,