Fix orjson loading error

[emilykl]

Closes #3567
Also addresses plotly/dash#2797

Summary

Address the error described in #3567 by adjusting the module loading logic in get_module.

The error can be reproduced by following these steps.

As best I can tell, the error is caused by a race condition between the multiple threads in the Flask hot reloader, and the fact that Python adds a module to sys.modules before the module has been executed [1] [2]. If one thread attempts to load orjson after it has been added to sys.modules but before it has been executed, we'll get the error.

Fix

• Change logic in get_module so that when should_load=True, we just call import_module right away instead of checking sys.modules first.
• This resolves the issue because import_module contains logic to verify that the module is fully initialized
• Logic for when should_load=False remains the same: If the module is in sys.modules we return it, otherwise we return None

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[T4rk1n]

The else is not necessary after return.

[T4rk1n]

💃

[alexcjohnson]

💃 very nice diagnosis & fix!

[archmoj]

Some tests are failing on CI.
Over to @LiamConnors 🙏