build+docs: use Python 3.11 and pin a recent version of pytest

[gvwilson]

• conda create -n plotly-dev python currently gives Python 3.12.3.
• pytest -v packages/python/plotly/plotly/tests/ then fails with ModuleNotFoundError: No module named 'imp'.
  • pip install imp fails: the imp module was deprecated in Python 3.11 and has been removed in Python 3.12.
• So update contributing.md to specify conda create -n plotly-dev python=3.11
• But now pytest fails
  • File "<frozen importlib._bootstrap>", line 1072, in _find_spec
  • AttributeError: 'AssertionRewritingHook' object has no attribute 'find_spec'
• Problem was pytest==3.5.1 in packages/python/plotly/optional-requirements.txt
• Update to pytest==8.1.1 and pytest runs
  • Some tests are failing (orca problems, statsmodels not installed, etc.)
  • Will fix these in separate PRs
• Note: updating pytest also solves the problem with the imp module not being available in Python 3.12.

Code PR

• [X] 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.
• [X] I have added tests (if submitting a new feature or correcting a bug) or modified existing tests.
• [X] I have added a CHANGELOG entry if fixing/changing/adding anything substantial.

closes #4591

[gvwilson]

@emilykl not yet - I'd like to fix the broken tests on Python 3.11 first, then switch back to Python 3.12 and tackle the imp issue. Good news is that pyzmq 26.0.3 seems to install with Python 3.12.

[gvwilson]

Results on MacOS Sonoma 14.4.1 - I will update my PR tomorrow to (a) pin the pytest version and (b) modify contributing.md to say this has been tested on Python 8-11. Still have failing tests in several areas; I'll dig into those as well in separate PRs.

Python version	existing pytest==3.5.1	updated pytest==8.1.1
3.8.19	fails: ImportError: cannot import name 'Config' from 'pytest'	succeeds
3.9.19	fails: ImportError: cannot import name 'Config' from 'pytest'	succeeds
3.10.14	fails: TypeError: required field "lineno" missing from alias	succeeds
3.11.9	fails: TypeError: required field "lineno" missing from alias	succeeds

[gvwilson]

PR updated: @LiamConnors can you please take a look at contributing.md and CHANGELOG.md and let me know?

[gvwilson]

@LiamConnors pushed a fix for the CHANGELOG (sorry about 8-11 instead of 3.8-3.11) - rebased to put it in the same commit as the other changes to hide my silly typo.