remove use of assignment expression for greater portability

[ericniebler]

Description

pr #3180 added an assignment expression (a := b) to a python script that is used to build the docs. assignment expressions are a relatively new addition to python and are not supported by the version of python used in ci to build the docs.

this pr replaces the assignment expression with an ordinary assignment.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[shwina]

Thanks - the new failures are now due to the use importlib.files which was introduced in Python 3.9.

Annoyingly, it's not our build or test environment that is running up to this Python version limitation (in fact, for example, cuda_parallel requires Python >= 3.9), but rather the docs build environment which uses Python 3.7.

Python 3.7 is currently well past its EOL.

I'm investigating whether/how we might be able to update the Python version in our docs env. If I can't find a way to do that by tomorrow, we can update this PR to make the rest of the code base compliant with Python 3.7. My understanding is that in the longer term, @alliepiper is looking into an overhaul of the documentation infrastructure anyhow.

[github-actions]

🟩 CI finished in 28m 09s: Pass: 100%/1 | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s

• 🟩 python: Pass: 100%/1 | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s

  🟩 cpu
    🟩 amd64              Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 ctk
    🟩 12.6               Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 cudacxx
    🟩 nvcc12.6           Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 cudacxx_family
    🟩 nvcc               Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 cxx
    🟩 GCC13              Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 cxx_family
    🟩 GCC                Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 gpu
    🟩 v100               Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  🟩 jobs
    🟩 Test               Pass: 100%/1   | Total: 28m 09s | Avg: 28m 09s | Max: 28m 09s
  

👃 Inspect Changes

Modifications in project?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
	CUDA Experimental
+/-	python
	CCCL C Parallel Library
	Catch2Helper

Modifications in project or dependencies?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
	CUDA Experimental
+/-	python
	CCCL C Parallel Library
	Catch2Helper

🏃‍ Runner counts (total jobs: 1)

#	Runner
1	linux-amd64-gpu-v100-latest-1