09_git.ipynb: Make it work in Python 3.6, and other fixes

The `capture_output` named parameter was added to `subprocess.run` in
Python 3.7.  For GitHub CI (which uses Python 3.6) to work its use
had to be replaced by using `stdout=PIPE`.

While at it, emphasize where tests are in Markdown cells, and fix
outdated information about the repository used for tests.

Author: jnareb

09_git.ipynb

@@ -3322,7 +3322,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Let's test `sparse_clone()`, for example using very small [`https://github.com/octocat/Spoon-Knife`](https://github.com/octocat/Spoon-Knife) repository created for the purpose of teaching forking in GitHub."
+    "Let's **test** **`sparse_clone()`**, for example using quite small and archived [`https://github.com/githubtraining/hellogitworld`](https://github.com/githubtraining/hellogitworld) \"Hello Git World\" sample training repository."
    ]
   },
   {
@@ -3361,8 +3361,10 @@
     "\n",
     "#!git -C repos/hellogitworld.git config --get remote.origin.url\n",
     "#!git -C repos/hellogitworld.git remote show -n origin\n",
+    "\n",
+    "# NOTE: `capture_output` was added in Python 3.7, GitHub CI runs Python 3.6\n",
     "assert subprocess.run(['git', '-C', repo_path, 'config', '--get', 'remote.origin.url'],\n",
-    "                      capture_output=True).stdout.decode(\"utf-8\").strip() == repo_url\n",
+    "                      stdout=subprocess.PIPE).stdout.decode(\"utf-8\").strip() == repo_url\n",
     "print('ok - cloned repository has correct url')\n",
     "\n",
     "# attempt cleanup after the test, which may not work on MS Windows\n",
@@ -3522,7 +3524,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Let's test the easy-to-test internal function `_repo_name_to_out_path()`"
+    "Let's **test** the easy-to-test internal function **`_repo_name_to_out_path()`**."
    ]
   },
   {
@@ -3671,7 +3673,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Instead of testing all those functions one by one, let's simply test the final one, namely `commit_graph()`, which would would as an integration test.  It would stress almost all other functions."
+    "Instead of testing all those functions one by one, let's simply **test the final one**, namely **`commit_graph()`**, which would would as an integration test.  It would stress test almost all other functions."
    ]
   },
   {