[llvm][TableGen][Jupyter] Record current python when kernel is installed

[DavidSpickett]

Summary

Previously the kernel.json would always point to python3 even if you
installed using a python from a virtualenv. This meant that tools like VSCode
would try to run the kernel against the system python and fail.

Added a note to the readme about it. I've removed the need to
add to PYTHONPTHON as well, turns out it wasn't needed.

This fixes an issue reported in https://discourse.llvm.org/t/tablegen-the-playground-ipynb-file-is-not-working-as-expected/71745.

[DavidSpickett]

Remove the very proffesional runtime error I was using to check PYTHONPATH wasn't needed :).

[awarzynski]

LGTM, thanks!

llvm/utils/TableGen/jupyter/README.md

 [sql_query_backend.ipynb](sql_query_backend.ipynb) - How to write a backend using
 JSON output and Python.
 
 Notebooks can be viewed in browser on Github or downloaded and run locally. If
 that is not possible, there are Markdown versions next to the notebook files.
 
 ## TableGen Kernel
 
-To use the kernel, first install it into jupyter:
+To use the kernel, first install it into jupyter.
 
-    python3 -m tablegen_kernel.install
-
-Then put this folder on your PYTHONPATH so jupyter can find it:
+If you have installed Jupyter into a virtual environment, adjust `python3` to
+be the interpreter for that environment. This will ensure that tools run the
+kernel in the correct context.
 
-```shell
-    export PYTHONPATH=$PYTHONPATH:<path to this dir>
-```
+    python3 -m tablegen_kernel.install
 
 Then run one of:
 
 ```shell
     jupyter notebook
     # Then in the notebook interface, select 'LLVM TableGen' from the 'New' menu.
 
     # To run the example notebook in this folder.

llvm/utils/TableGen/jupyter/tablegen_kernel/assets/kernel.json

-{
-    "argv": [
-        "python3", "-m", "tablegen_kernel", "-f", "{connection_file}"
-    ],
-    "display_name": "LLVM TableGen",
-    "language": "tablegen",
-    "language_info": {
-        "name": "tablegen",
-        "codemirror_mode": "tablegen",
-        "mimetype": "text/x-tablegen",
-        "file_extension": ".td",
-        "pygments_lexer": "text"
-    }
-}

llvm/utils/TableGen/jupyter/tablegen_kernel/install.py

 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 import os
+import json
+import sys
 import argparse
+from tempfile import TemporaryDirectory
 from jupyter_client.kernelspec import KernelSpecManager
 
 
 def install_my_kernel_spec(user=True, prefix=None):
     """Install the kernel spec for user in given prefix."""
     print("Installing llvm-tblgen IPython kernel spec")
-    pkgroot = os.path.dirname(__file__)
+
+    kernel_json = {
+        "argv": [
+            sys.executable, "-m", "tablegen_kernel", "-f", "{connection_file}"
+        ],
+        "display_name": "LLVM TableGen",
+        "language": "tablegen",
+        "language_info": {
+            "name": "tablegen",
+            "codemirror_mode": "tablegen",
+            "mimetype": "text/x-tablegen",
+            "file_extension": ".td",
+            "pygments_lexer": "text"
+        }
+    }
+
+    with TemporaryDirectory() as tmpdir:
+      json_path = os.path.join(tmpdir, "kernel.json")
+      with open(json_path, 'w') as json_file:
+        json.dump(kernel_json, json_file)
       KernelSpecManager().install_kernel_spec(
-        os.path.join(pkgroot, "assets"), "tablegen", user=user, prefix=prefix
+          tmpdir, "tablegen", user=user, prefix=prefix
       )
 
 
 def _is_root():
     """Returns whether the current user is root."""
     try:
         return os.geteuid() == 0
     except AttributeError:
         # Return false wherever unknown.