"Create a transpiler plugin" uses Python >3.9 features

[frankharkins]

The notebook docs/transpile/create-a-transpiler-plugin.ipynb is failing in https://github.com/Qiskit/documentation/actions/runs/9485973981/job/26139208772?pr=1537:

 Cell In[1], line 12, in MyLayoutPlugin()
      8 class MyLayoutPlugin(PassManagerStagePlugin):
      9     def pass_manager(
     10         self,
     11         pass_manager_config: PassManagerConfig,
---> 12         optimization_level: int | None = None,
     13     ) -> PassManager:
     14         layout_pm = PassManager(
     15             [
     16                 VF2Layout(
   (...)
     22             ]
     23         )
     24         layout_pm += common.generate_embed_passmanager(pass_manager_config.coupling_map)

TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

This is because that syntax was introduced in Python 3.10, but we test the notebooks with Python 3.9 (as that's the lowest version Qiskit supports).

Options to fix this are:

• a) Replace int|None with Optional[int] (my preferred choice)
• b) Use from __future__ import annotations
• c) Decide to test notebooks on Python 3.10 or newer

[Eric-Arellano]

We should do either option 1 or 2. It's useful for users if the notebook works with Python 3.9 because they might be using 3.9 locally. I'm fine with option 1.

[frankharkins]

Plot thickens: It already has the future import...

[frankharkins]

This was fixed in #1546