doc: fix skipping certain output types

Fixes #168

Without this fix we have the following error:

```
py.test --nbval notebooks/hummingbird.ipynb --sanitize-with notebooks/output-sanitize.cfg
================================================================ test session starts =================================================================
platform linux -- Python 3.7.10, pytest-6.2.3, py-1.10.0, pluggy-0.13.1
rootdir: /repos/PAVICS-e2e-workflow-tests
plugins: tornasync-0.6.0.post2, anyio-2.2.0, nbval-0.9.6, dash-1.20.0
collected 0 items
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/_pytest/main.py", line 269, in wrap_session
INTERNALERROR>     session.exitstatus = doit(config, session) or 0
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/_pytest/main.py", line 322, in _main
INTERNALERROR>     config.hook.pytest_collection(session=session)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
INTERNALERROR>     return outcome.get_result()
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/_pytest/main.py", line 333, in pytest_collection
INTERNALERROR>     session.perform_collect()
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/_pytest/main.py", line 620, in perform_collect
INTERNALERROR>     rep = collect_one_node(self)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/_pytest/runner.py", line 457, in collect_one_node
INTERNALERROR>     ihook.pytest_collectstart(collector=collector)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
INTERNALERROR>     return outcome.get_result()
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/opt/conda/envs/birdy/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/repos/PAVICS-e2e-workflow-tests/conftest.py", line 2, in pytest_collectstart
INTERNALERROR>     collector.skip_compare += 'text/html', 'application/javascript',
INTERNALERROR> AttributeError: 'Session' object has no attribute 'skip_compare'

=============================================================== no tests ran in 0.01s ================================================================
```

Author: tlvu

docs/source/index.ipynb

@@ -568,7 +568,8 @@
    "outputs": [],
    "source": [
     "def pytest_collectstart(collector):\n",
-    "    collector.skip_compare += 'text/html', 'application/javascript', 'stderr',"
+    "    if collector.fspath and collector.fspath.ext == '.ipynb':\n",
+    "        collector.skip_compare += 'text/html', 'application/javascript', 'stderr',"
    ]
   }
  ],
@@ -588,7 +589,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.6"
+   "version": "3.7.10"
   }
  },
  "nbformat": 4,